Refactor Crystal::Exception to Crystal::CodeError

[straight-shoota]

This patch introduces Crystal::Error as base class for all compiler errors and renames Crystal::Exception to Crystal::CodeError.

Crystal::LocationlessException as a more specific error type is unnecessary. The relationship is now reversed: CodeError < Error (previously: LocationlessException < Exception) and Crystal::Error is used as exception type for non code-related errors.

[Fryguy]

Naming-wise, if the Crystal:: namespace implies "compiler", then Crystal::Error seems good otherwise I'd suggest something like Crystal::CompilerError or Crystal::CompilationError

[straight-shoota]

@Fryguy Good call. Indeed Crystal namespace is also used by the language runtime. But I don't think this overlap is a problem, at least not for this PR.