Print meaningful error messages with code location

[sonmarcho]

For now, whenever Aeneas encounters an error it simply crashes. It would be better to print an error message indicating the reason behind the error and most importantly the part of the code which triggered it.

A first step might simply be to print the file name and the location inside the file.

We also want to be able to run Aeneas in two modes, like Charon:

• do not abort on failure, go as far as possible and eventually generate definitions with holes while reporting errors (better for the user)
• abort on the first error (better for debugging)

Later we might want to have a Rust executable which links itself to the Rust compiler, runs Charon then Aeneas and reports all the error messages in the same session.

[sonmarcho]

Mostly solved by #95