Fixed stack trace of task runner exception being discarded

This should fix the cause being discarded when tasks fail. In order to debug configuration issues, it's useful to know exactly what went wrong. In the current implementation, the only error you'll ever see is: `'gulp build --no-color' failed`, which is not terribly helpful when it comes to determining the root cause of an issue.

This patch should print an error including the complete stack trace of the cause, for example:
```
[ERROR] Failed to execute goal com.github.eirslett:frontend-maven-plugin:0.0.16-SNAPSHOT:gulp (gulp-build) on project proposal-tool-frontend: Failed to run task: 'gulp build --no-c
olor' failed. java.io.IOException: Cannot run program "c:\bla\bla\src\node\node" (in directory "c:
\blabla\src"): CreateProcess error=2, The system cannot find the file specified -> [Help 1]
```

Using the `-X` Maven parameter outputs the entire stack trace, as expected.