* Module Cerrors is now called Diagnostic and can be used in parts of CompCert other than cparser/

* Replaced eprintf error.  Instead of having eprintf msg; exit 2 use the functions from the
Diagnostics module.

* Raise on error before calling external tools.

* Added diagnostics to clightgen.

* Fix error handling of AsmToJson.

* Cleanup error handling of Elab and C2C.

*The implementation of location printing (file & line) is simplified and correctly prints valid filenames with invalid lines.