#214 Give better messages on malformed tables

[Jynn Nelson]

This does several things:
- Specifies Misaligned right border for that error, instead of just "malformed table".
- Shows the line where each error happened, not the line where the table starts.
- This had a complication that line numbers appear to be wrong when include directives are present (they include the lines in the source document, instead of being relative to the included document). Just disabled the new smarter logic in that case.
- Changes malformed_table to require both detail and an offset, so poor errors like this can't happen in the future.

Fixes https://sourceforge.net/p/docutils/bugs/504/.

[Günter Milde]

Thank you for the patch. I will have a look at this once 0.22 is out. (We don't want delay the release any further by new changes to the release candidate unless it is urgent.)