Runtime errors name the line but not the file #1657

Runtime error messages give a line but no file name (Error line 12: ...). In any multi-file program (import, load_file, the guest front ends, packages) the reader cannot tell which file the line is in. The standard form is file:line: msg. EigenLua has to add positions itself to produce Lua's prog.lua:12: msg. This was noted during EigenLua round 1 and left unfiled; filed now.

Done when

• Uncaught errors and e.message/e.line carry the source file, in a standard file:line form for errors from imported or loaded files.
• The goldens that change are updated in the same PR, and the change is listed as breaking in the changelog (batched with a Value-model release: booleans, then integers, strings and map keys in ONE breaking release #1644 release if it changes e.message).