Fixing hacky exception handling and bug in import fixer

Duplicate of !152 (closed) because of merge issues.

Hacky exception handling by investigating the exception's class it not necessary anymore.

A bug in the ImportFixer reported by Kasra that happens with aliased imports (import something as something_else, for example) was fixed too and a test added to cover this case.

This MR fixes bugs:

• https://nexedi.erp5.net/bug_module/20160708-166C255
• https://nexedi.erp5.net/bug_module/20160708-1270DEA