custom_target: catch and reject input files that do not exist

Currently there is a try/except around the function that detects and
rejects this, which instead of rejecting it, spawns a warning and
continue.

This warning exists because of 'test cases/vala/9 gir/' which passes a
vala generated output that isn't a return value (!!!) using string
joining with the meson.current_build_dir() function (also !!!) because
we officially document this (!!! for a third time) as the only way to
make a vala shared library generate a typelib with a custom_command from
the automatically generated gir:
https://mesonbuild.com/Vala.html#gobject-introspection-and-language-bindings

In #3061 we converted strings to Files, but only if none of them were
this vala hack. Due to the precise implementation, we also failed to
convert strings to Files if any other error occurred, but since we only
want to ignore errors for generated vala outputs, tighten that check and
specifically call out generated files in the warning.

Fixes #8635

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