Catch symdiff errors.

[thorstenhater]

• Make symbolic differentiation return a more refined type.
• Catch said error when trying to test for linearity
• Catch the propagated error in CNExp and recommend using a different one.
• Straighten out the error printing at top level
• Add regression tests

Closes #1893

New output show the full error stack

bin/modcc bug_1893/bug_1893.mod -N 'arb::allen_catalogue' -t cpu -t gpu
error trace:
  * bug_1893/bug_1893.mod:(line 27,col 26) symbolic differential of unrecognized binary expression
  * bug_1893/bug_1893.mod:(line 27,col 10) CNExp: Could not determine linearity, maybe use a different solver?

[boeschf]

Looks good, but please consider changing the error message

[thorstenhater]

@boeschf I fear you to r'approve.