Improve error handling for missing source files #14037

Job	Run time
Test mypyc suite with 32-bit Python	6m 58s
Test suite with py38-ubuntu, mypyc-compiled	18m 50s
Test suite with py38-windows-64	20m 25s
Test suite with py39-ubuntu	17m 20s
Test suite with py310-ubuntu	17m 3s
Test suite with py311-ubuntu, mypyc-compiled	16m 42s
Test suite with py312-ubuntu, mypyc-compiled	18m 43s
Test suite with py313-ubuntu, mypyc-compiled	18m 38s
mypyc runtime tests with py39-macos	6m 24s
Type check our own code (py38-ubuntu)	1m 53s
Type check our own code (py38-windows-64)	2m 31s
Formatting and code style with Black + ruff	54s
	2h 26m 21s

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