Gracefully deal with unimplemented language elements.

[FriedrichGretz]

Is your feature request related to a problem? Please describe.
Whenever the language server encounters a language element that has not yet been implemented it simply crashes, forwarding the compiler's error message.

For example:

Exception in language server Print, Assert, Assume not implemented yet.

Describe the solution you'd like
Display this issue like any other expected syntax error.

[schorg]

failwith is no longer used in the blech compiler to report unsupported language features.
See commit boschresearch/blech@fb65953