Include template name in render error SourceLocation

[ezralicodes]

Summary

When an error is raised while rendering an {% include %} / {% extends %} template, the line/column in SourceLocation refer to that nested file, but the message did not say which template it was.

This stores the template path/key on Template and SourceLocation, and includes it in the exception text when present:

[inja.exception.render_error] (at 1:10 in header.html) variable 'name' not found

Anonymous string templates keep the previous (at line:col) format.

Fixes #249

Test plan

• Added include-files / error reports included template name regression
• Existing renderer/parser throw-string checks still match (empty filename → unchanged message)