cmd/go/internal/cache: return and check errors from Trim
It's fine to ignore errors when reading trim.txt, since it might not
exist or might be corrupted. However, if we encounter an error in
writing the file, we will end up trimming again at every cmd/go
invocation, which will cause invocations to become progressively
slower (because each command will check more and more cache files for
trimming).
Although that situation would not cause the output of any 'go' command
to be invalid, it still seems better to escalate the problem to the
user to be fixed instead of proceeding in a degraded state.
Returning an explicit error also allows TestCacheTrim to skip if the
Trim error indicates that a required operation (in this case, file
locking) is not supported by the platform or filesystem.