x/tools/gopls: improve instrumentation for corrupt export data during import

[findleyr]

Telemetry data at https://storage.googleapis.com/prod-telemetry-merged/2023-10-28.json clearly demonstrates a package that cannot be imported: there are in fact two bug reports, one each in type checking (#64235) and analysis (#64236).

Clearly some invariant is being violated. We should use the reportf callback passed into the importer to get a more accurate stack, as otherwise this is unlikely to be actionable (we can't see the error message in telemetry, which we probably didn't consider when adding this instrumentation).

Putting this in the v0.14.2 milestone as it would be nice to get to the bottom of this error. I'll push down the instrumentation.

CC @adonovan