error messages and categories

This merge request introduces a number of helpful error/warn messages that point to confusing misconfigurations.

Also, !1366 (merged) makes it so categories are now defined by the repo. Categories can be defined in the config so that lint has a list of categories to enforce. This also provides a place for localization and icons for the categories. The old way of defining categories was just listing them in app metadata files. This restores that way of functioning when using index-v2.

closes #1137 (closed) fdroidclient#2619 (comment 1421280589)

This touches issues/suggestions from @pmmayero @grote @jspricke @FestplattenSchnitzel @obfusk @uniqx.