Improve error checking and reporting in Eve code generation.

[egparedes]

Improve error checking and reporting in Eve code generation.

Main changes:

• Raising custom exceptions in template rendering errors with proper information
• Raising custom exceptions in source formatting errors
• Setting StrictUndefined jinja2 option
• Replacing FormatTemplate implementation strategy with full support for f-string expressions
• Converting Template into a typing.Protocol (supporting structural subtyping for Template implementations)
• Remove obsolete utils.XStringFormatter class