refactor[next] error handling

[petiaccja]

Contents:

• Add CompilerError and subclasses for specific errors
  • Location info + error msg
• Removed old GT*Error classes, DialetSyntaxError classes
• Replaced most raise statements in frontend [needs further work]
  • Most error messages now have proper location information
  • Some of the errors have updated types and error messages
• Replaced raise statements in type_system [needs further work]
• Added sys.excepthook for printing uncaught CompilerErrors
• Added __str__ method for printing uncaught errors in PyCharm

Usage:

• Compilation errors: raise one of the classes from next.errors (alias of next.errors.exceptions) or create a new class if makes sense
• General errors: use Python's builtin exception classes. If necessary, new classes can be added somewhere in next.errors.
• Don't override sys.excepthook!

Results:

Errors are displayed similarly to Python syntax errors:

Source location (most recent call last):
  File "/home/petiaccja/Work/ETHZ/code/gt4py_petiaccja/tests/next_tests/exception_printing.py", line 23
    def foo(var):
            ^^^
gt4py.next.errors.exceptions.MissingParameterAnnotationError: parameter 'var' is missing type annotations

[egparedes]

The main changes in the PR look good to me. I've left many comments, questions and suggestions, but usually minor things.

[egparedes]

All the coding and design issues have been addressed. I only have comments about coding style. I'll approve the PR as soon as they get addressed.

[egparedes]

ILGTM.