Review how NMODL crashes if sympy isn't installed.

[1uc]

Rumour has it that NMODL crashes very inelegantly when trying to use sympy when it's not installed. The task is to check how NMODL crashes when it can't find sympy.

Context: #1259 which means certain exceptions are tolerated, and interpreted as "it already ran". The worry/rumour is that it (or something else) is causing bad error messages for certain genuine problems, like failure to import sympy.