Add missing log statement.

Without this change, any emitted response exception was not logged.
Add a configuration UI checkbox for the new ignoreUnknownIssueProjectKeys setting.
Handle the 404 responses emitted for missing project keys.
Exclude issues with invalid project keys from all validation.

This fixes a bug in the previous implementation, whereby a commit would
be treated as valid due to validation not being performed on issues with
an invalid project key, but the list of matched issues being non-empty.

Since the list was non-empty, the check for *any* matched issue
succeeded, and the array of errors was empty: the commit was considered
valid.