Add more validations to .mbtiles detection

[nyurik]

As part of mbtiles type detection, we should catch some common issues:

• detect if tiles (simple) or map (deduplicated) tables have an index by z,x,y columns
• ???

In case of a problem, use a warn! macro to print a warning, unless we are certain the issue makes it impossible to use a file, in which case it should be appropriate error (add it to the error enum).

A bigger discussion is if mbtiles should stop using logging, and return some Vec<MbtilesWarning> if non-critical issues are detected.