ImportError of openbabel in cmdline gives wrong errormessage

[cstein]

When openbabel can't be found by FragIt, the error message is that FragIt cannot find itself. Thus, it hides the true problem.

The problem is that in the fragit executable everything is masked with a

try:
    ...
except ImportError:

but not being able to import openbabel raises an import error. It should fail more gracely. Perhaps it's time to make a FragItException class.

[cstein]

Related to #12