[LCGS] Add compile errors

[NicEastvillage]

Currently the program crashes with a sometimes-helpful message, whenever the LCGS program is faulty. Ideally it should give a (or multiple) compiler errors describing exactly what is wrong.

Additionally, the compile errors can be improved immensely if we calculate the spans of AST nodes. That is, the start and end position of the text that resulting in the AST node. Span is already defined in lcgs::parse.rs as well as a helper function to capture the spans of parsed text.

[NicEastvillage]

I found a great article that will help us in this endeavour: https://eyalkalderon.com/blog/nom-error-recovery/