core: fix description of "resources" service error (#3119)

core: fix description of "resources" service error (#3119)

The "resources" error is really just the generic error we return when
we hit some kind of error and we have no more appropriate error for the case to
return, for example because of some OS error.

Hence, reword the explanation and don't claim any relation to resource limits.

Admittedly, the "resources" service error is a bit of a misnomer, but I figure
it's kind of API now.

Fixes: #2716