Slightly improved exception handling for wrong dotGitDirectory.

Hello Konrad,

To make stack trace as readable as possible when incorrect `dotGitDirectory` is passed I would add just one more simple improvement. I know it increases the code a little bit, but the result is

	[INFO] Scanning for projects...
	[INFO]                                                                         
	[INFO] ------------------------------------------------------------------------
	[INFO] Building rest-showcase 1.0-SNAPSHOT
	[INFO] ------------------------------------------------------------------------
	[INFO] 
	[INFO] --- git-commit-id-plugin:2.1.16-SNAPSHOT:revision (default) @ rest-showcase ---
	[info] dotGitDirectory C:\Work\Projects\rest-showcase\src
	[info] git.build.user.name Tomasz Przybyla
	[info] git.build.user.email bsodzik@tlen.pl
	org.apache.maven.plugin.MojoExecutionException: Could not get HEAD Ref, are you sure you've set the dotGitDirectory property of this plugin to a valid path?
		at pl.project13.maven.git.JGitProvider.prepareGitToExtractMoreDetailedReproInformation(JGitProvider.java:104)
		at pl.project13.maven.git.GitDataProvider.loadGitData(GitDataProvider.java:57)
		at pl.project13.maven.git.GitCommitIdMojo.loadGitDataWithJGit(GitCommitIdMojo.java:485)
		at pl.project13.maven.git.GitCommitIdMojo.loadGitData(GitCommitIdMojo.java:457)
		at pl.project13.maven.git.GitCommitIdMojo.execute(GitCommitIdMojo.java:315)
		at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:106)
		at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:208)
		at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
		at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
		at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:84)
		at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:59)
		at org.apache.maven.lifecycle.internal.LifecycleStarter.singleThreadedBuild(LifecycleStarter.java:183)
		at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:161)
		at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:317)
		at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:152)
		at org.apache.maven.cli.MavenCli.execute(MavenCli.java:555)
		at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:214)
		at org.apache.maven.cli.MavenCli.main(MavenCli.java:158)
		at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
		at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
		at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
		at java.lang.reflect.Method.invoke(Method.java:483)
		at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
		at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
		at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
		at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)
		at org.codehaus.classworlds.Launcher.main(Launcher.java:46)
		at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
		at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
		at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
		at java.lang.reflect.Method.invoke(Method.java:483)
		at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)
	[INFO] ------------------------------------------------------------------------
	[INFO] BUILD FAILURE
	[INFO] ------------------------------------------------------------------------
	[INFO] Total time: 1.995s
	[INFO] Finished at: Fri Nov 28 08:22:02 CET 2014
	[INFO] Final Memory: 12M/176M
	[INFO] ------------------------------------------------------------------------
	[ERROR] Failed to execute goal pl.project13.maven:git-commit-id-plugin:2.1.16-SNAPSHOT:revision (default) on project rest-showcase: Could not complete Mojo execution... Could not get HEAD Ref, are you sure you've set the dotGitDirectory property of this plugin to a valid path? -> [Help 1]
	[ERROR] 
	[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
	[ERROR] Re-run Maven using the -X switch to enable full debug logging.
	[ERROR] 
	[ERROR] For more information about the errors and possible solutions, please read the following articles:
	[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException

You are the boss here, yours is the decision :)
BTW. Thanks for great plugin - with Spring Boot it works like a charm!