narrow the exception types and pretty print exceptions

[thautwarm]

Currently, the exceptions are not well-rendered, and I just collect the error here,

urgent-lang/urgent/compiler.py

Lines 81 to 85 in 1fc741a

	self._loc = expr.loc
	try:
	ret = self.dispatches[expr.__class__](self, expr)
	return ret
	except Report:

,
and do some basic formatting at

urgent-lang/urgent/repl1s.py

Lines 62 to 64 in 1fc741a

	except Report as e:
	if debug:
	traceback.print_exc(limit=tracing_limit)

.

Also, many exceptions raised when compiling are just this simple, without a proper exception type specified:

urgent-lang/urgent/compiler.py

Line 309 in 1fc741a

raise Exception("redefinition of operator properties")

To narrow the exception types, we might create a new module called excs, and make some new exception types in this module, for raising during compilation. The existing exception types might be migrated to excs module.

To pretty print the exceptions, we might create a method in excs to well render the exception to string message.