This acknowledges "too many connections" and "max user connections"
errors in MySQL and propagates the error up the chain so we can notify
the user and return the correct HTTP code.

This adds the `isIgnorableError` method and calls it when error checking
in `GetUserInvite()`, returning "not found" if the rror comes up.

This ensures the writefreely pkg can be used in other applications that
need to load mysql themselves -- this can be done by building with the
tag: wflib

Ref T613

This adds a new `sqlite` build tag that you should include only if you
want SQLite3 support built in. Both `make run` and `make release` create
builds with SQLite included.