Cleanup exception catching

No more `try with _ ->`. Should make sure C-c is correctly handled everywhere,
and we don't catch other unexpected errors.
New guideline on `try with`:
- either catch specific exceptions -- always the best
- or re-raise the same exception, for finalisers
- or call `OpamMisc.fatal e` on the exception before continuing
Cleanup warnings
Merge pull request #1147 from AltGr/code-cleanup

Fixed warnings and exception catching.