Improve error message when extension file does not contain module

[jakobnissen]

I just tried to create a package extension, but forgot to wrap the code in the extension file in its own module. I got an error saying:

[ Info: Precompiling TestExt [29286dec-c893-52d1-88fe-e07e79eb4405]
ERROR: LoadError: ArgumentError: Package Base does not have Test in its dependencies:
- You may have a partially installed environment. Try `Pkg.instantiate()`
  to ensure all packages in the environment are installed.
- Or, if you have Base checked out for development and have
  added Test as a dependency but haven't updated your primary
  environment's manifest file, try `Pkg.resolve()`.
- Otherwise you may need to report an issue with Base
Stacktrace:
 [1] macro expansion
   @ Base ./loading.jl:1770 [inlined]
 [2] macro expansion
   @ Base ./lock.jl:267 [inlined]
 [3] __require(into::Module, mod::Symbol)
   @ Base ./loading.jl:1747

IMO this error is pretty confusing because it claimed I was trying to load Test from Base, whereas in fact, I tried loading it in my extension module.

[IanButterworth]

Investigating this I also noticed that Pkg.precompile reports the failure but swallows the error

(@v1.11) pkg> precompile
Precompiling project...
  ✗ PGFPlotsX → ColorsExt
  0 dependencies successfully precompiled in 1 seconds. 53 already precompiled.

(@v1.11) pkg>