Fix issue #2319 (crash on import of candid class)

[crusso]

• errors, doesn't crash, on import of candid service class
• improves on UnsupportedCandid error reporting, adding source location and message.

[dfinity-ci]

This PR does not affect the produced WebAssembly code.

[crusso]

@chenyan-dfinity PTAL
BTW, do you know if we actually type-check (i.e. kind-check) the imported candid file before or during import? There might be a an unlikely, but possible, security hole if we don't.

[chenyan-dfinity]

Yes, we type check the did file when importing. https://github.com/dfinity/motoko/blob/master/src/pipeline/pipeline.ml#L337