Errors API

[Omar Tawfik]

Instead of an Error type, I suggest moving to Diagnsotic notation, popularized by Roslyn, TypeScript, VS Code, and others.

• Diagnostic::message() -> String to return a human readable string, usable in editors and downstream APIs.
• Diagnostic::kind() -> DiagnosticKind to return an enum: ParserError, ValidationError, etc...
• Diagnostic::code() -> String the string represenatation of DiagnosticKind, suitable to surface in editors and URLs (error glossary/wiki).
• Diagnostic::severity() -> DiagnosticSeverity to return an enum: Error, Warning, Info, etc... (same as LSP severity levels)
• Diagnostic::location() for file/range info.

As for the errors API:

• Remove ariadne reports from the API (should only be the CLI).
• CLI-specific error codes, and unified representation (like FileNotFound), instead of just panicking.