[SQL] Minor cleanup of comments, errors and override.

Author: Michael Armbrust <michael@databricks.com>

Closes #3257 from marmbrus/minorCleanup and squashes the following commits:

d8b5abc [Michael Armbrust] Use interpolation.
2fdf903 [Michael Armbrust] Better error message when coalesce can't be resolved.
f9fa6cf [Michael Armbrust] Methods in a final class do not also need to be final, use override.
199fd98 [Michael Armbrust] Fix typo