raise EasyBuildError rather than ImportError in only_if_module_is_available decorator

[boegel]

This ensures cleaner error messages in case a required Python package is not available, cfr. easybuilders/easybuild-easyconfigs#2670:

Now:

== temporary log file in case of crash /tmp/eb-uts8ZU/easybuild-nm6hF1.log
ERROR: No module named git; required module 'git' is not available (provided by Python package GitPython, available from https://pypi.python.org/pypi/GitPython)

was:

== temporary log file in case of crash /tmp/eb-3ioxWD/easybuild-oeMsuZ.log
Traceback (most recent call last):
  File "/home/adam/.local/easybuild/software/Python/2.7.10-foss-2015b/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/home/adam/.local/easybuild/software/Python/2.7.10-foss-2015b/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/adam/.local/easybuild/software/EasyBuild/2.6.0/lib/python2.7/site-packages/easybuild_framework-2.6.0-py2.7.egg/easybuild/main.py", line 381, in <module>
    main()
  File "/home/adam/.local/easybuild/software/EasyBuild/2.6.0/lib/python2.7/site-packages/easybuild_framework-2.6.0-py2.7.egg/easybuild/main.py", line 248, in main
    new_pr(orig_paths, title=options.pr_title, descr=options.pr_descr, commit_msg=options.pr_commit_msg)
  File "/home/adam/.local/easybuild/software/EasyBuild/2.6.0/lib/python2.7/site-packages/easybuild_framework-2.6.0-py2.7.egg/easybuild/tools/utilities.py", line 145, in error
    raise ImportError(msg)
ImportError: 'gitdb' could not be found in your PYTHONPATH; required module 'git' is not available (provided by Python package GitPython, available from https://pypi.python.org/pypi/GitPython)

cc @verdurin

[hpcugentbot]

EasyBuild framework unit test suite PASSed (see https://jenkins1.ugent.be/job/easybuild-framework-pr-builder/2850/console for more details).

This pull request is now ready for review/testing.

Please try and find someone who can tackle this; contact @boegel if you're not sure what to do.

[boegel]

@wpoely86: please review?

[wpoely86]

👍

[boegel]

Thanks for the review @wpoely86!