Handle better missing source files

I forgot to include a source file in the meson.build file in a DLang application (although the source file was there), and ninja-build said:
app_test@exe/source_app_application.d.o: In function `_D5app3app11application5Usage13onThemeChangeMFC7gobject9ParamSpec9ParamSpecC7gobject7ObjectG7ObjectGZv':

Which is quite cryptic.

I believe this is because it fails when linking, however it should detect these errors like it does when the file is actually not present.

Start by reproducing the D language case from the issue with a source file present but omitted from meson.build, then inspect how Meson generates ninja-build targets and handles the resulting linker failure. Done means the missing source file is detected with a clear diagnostic comparable to the absent-file case.

Written by the indexing model from the issue text.