fix for #10

Hi,

this is the ouput when protoc failes and version cannot be termined.

```
[WARNING] Command line option -npr is deprecated and will be removed in future Maven versions.
[INFO] Scanning for projects...
[INFO]                                                                         
[INFO] ------------------------------------------------------------------------
[INFO] Building test-protobuf 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO] 
[INFO] --- protobuf-maven-plugin:0.6.1-SNAPSHOT:run (generate-sources) @ test-protobuf ---
[INFO] Protobuf dependency version 2.4.1
[INFO]     FOOBAR
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.588s
[INFO] Finished at: Thu Jan 17 20:15:41 CET 2013
[INFO] Final Memory: 6M/176M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal com.github.igor-petruk.protobuf:protobuf-maven-plugin:0.6.1-SNAPSHOT:run (generate-sources) on project test-protobuf: 'protoc' failed. Exit code 1 -> [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
```