deposit_check: Actually store warning in deposit status detail
Prior to this commit, only rejected deposit were storing problem details. Now that we can have warnings even in case of 'verified' deposit, we need to store that details for post-analysis.
Note that this also fixes the docstring of the overall class which were out of date since the beginning (duplicated from another class).
Related to #3677 (closed) Depends on !280 (closed)
Migrated from D7210 (view on Phabricator)