Ensure cataloging execution path does not hide potential results

We've adjusted the java cataloger to prefer adding warnings over stopping execution mid-cataloging in order to maximize the number of artifacts found (https://github.com/anchore/syft/pull/348), we should continue this trend and take a close look at the remaining analyzer modules to ensure they follow suit.

Start by reviewing the linked Java cataloger change in PR #348, then inspect the remaining analyzer modules and their cataloging error paths. Done means analyzers add warnings and continue cataloging where possible instead of stopping execution, while maximizing the artifacts found.

Written by the indexing model from the issue text.