Report errors in import statements.

[Kripner]

Solves the problem that invalid import statements were not reported as errors.

Before:

{"cmd": "import BlahBlahBlah"}

{"env": 0}

After:

{"cmd": "import BlahBlahBlah"}

{"messages":
 [{"severity": "error",
   "pos": {"line": 1, "column": 0},
   "endPos": null,
   "data":
   "unknown module prefix 'BlahBlahBlah'\n\nNo directory 'BlahBlahBlah' or file 'BlahBlahBlah.olean' in the search path entries:\n././.lake/packages/Cli/.lake/build/lib\n././.lake/packages/batteries/.lake/build/lib\n././.lake/packages/Qq/.lake/build/lib\n././.lake/packages/aesop/.lake/build/lib\n././.lake/packages/proofwidgets/.lake/build/lib\n././.lake/packages/importGraph/.lake/build/lib\n././.lake/packages/LeanSearchClient/.lake/build/lib\n././.lake/packages/plausible/.lake/build/lib\n././.lake/packages/mathlib/.lake/build/lib\n././.lake/build/lib\n/home/m/.elan/toolchains/leanprover--lean4---v4.15.0/lib/lean\n/home/m/.elan/toolchains/leanprover--lean4---v4.15.0/lib/lean"}],
 "env": 0}

Similarly, invalid import statements were not reported when processing a file using {"path": "..."}, which is now fixed.

[kim-em]

It seems that there is much more here than in the PR description. Do you want to push again with just the code you want here?