Report errors in import statements

[Kripner]

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

Before:

{"cmd": "import NonExistentImport"}

{"env": 0}

After:

{"cmd": "import NonExistentImport"}

{"messages":
 [{"severity": "error",
   "pos": {"line": 1, "column": 0},
   "endPos": null,
   "data":
   "unknown module prefix 'NonExistentImport'\n\nNo directory 'NonExistentImport' or file 'NonExistentImport.olean' in the search path entries:\n/home/m/.elan/toolchains/leanprover--lean4---v4.27.0-rc1/lib/lean"}],
 "env": 0}

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