#681 PLCgen: Improve prechecking error reports.

This patch has more precise error messages if possible, rather than the very generic "you didn't do one of the following 4 things". Also, it's better at indicating the point of the error (at the operator rather than the non-continuous side of the binary expression).

Other changes:

• Only one 'tau' event violation.
• Have both cont OP value and value OP cont comparisons, since now the error message is matching the code pattern.

It looks much less confusing to me now.

Closes #681 (closed)