Fix crash when passing missing file to zig test

[quantumhu]

Resolves #20954

Error message looks like this:

$ zig test tt.zig                                   
error: unable to load file './tt.zig': FileNotFound

$ zig test ./tt.zig
error: unable to load file './tt.zig': FileNotFound

$ zig test /test-folder/tt.zig 
error: unable to access directory '/test-folder': FileNotFound

$ zig test ./test-folder/tt.zig
error: unable to access directory 'test-folder': FileNotFound

[squeek502]

This is a possible fix, but it feels like the wrong fix. Using access to check for the existence of a file is something Zig tries to avoid.

The underlying problem as far as I can tell is that addModuleErrorMsg was made to depend on the AST of the file, but obviously there's no AST for a file that doesn't exist. The dependency on the AST was introduced in ded5c75 (cc @mlugg).

[mlugg]

Ah, yes, we will need to bypass the AST code path for whole file errors. Sorry!