Use case:

zig build-exe non_existent_file.zig

Previous behavior:

error.FileNotFound, followed by an error return trace

Behavior after this commit:

error: unable to read non_existent_file.zig: FileNotFound
(end of stderr, exit code 1)

This turns AllErrors.Message into a tagged union which now has the
capability to represent both "plain" errors as well as source-based
errors (with file, line, column, byte offset). The "no entry point found"
error has moved to be a plain error message.