* Log database connection error before humanizing it

The original, non-humanized error message may contain clues regarding the
error state, that might get lost in the humanization process.

* Log complete error at error log level

* Remove trailing colons from headings

* Reformat heading

* Add MySQL authentication troubleshooting

* Typo

* Reference MySQL 8 caveat in new MySQL section of admin guide

* Fix URL to troubleshooting guide section

* Update MySQL admin guide text

* "Fix" troubleshooting link