Better exception message when creating a new instance fails

With this patch also the message of InvocationTargetException cause is
included in the wrapping RuntimeException message.

Before:
```
java.lang.RuntimeException: Unable to instantiate Drone via org.openqa.selenium.firefox.FirefoxDriver(Capabilities)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
...
        at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
        at org.openqa.selenium.remote.HttpCommandExecutor$EntityWithEncoding.<init>(HttpCommandExecutor.java:420)
...
        at org.openqa.selenium.remote.RemoteWebDriver.startSession(RemoteWebDriver.java:216)
        at org.openqa.selenium.remote.RemoteWebDriver.<init>(RemoteWebDriver.java:111)
        at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:188)
...
```
Why the heck doesn't Firefox start?!

After:
```
java.lang.RuntimeException: Unable to instantiate Drone via org.openqa.selenium.firefox.FirefoxDriver(Capabilities): java.lang.NoClassDefFoundError: org/apache/http/entity/ContentType
```
Rest of the stack trace is not important any more. Now I know a class is missing and there's something wrong with my dependencies.