Change "cannot build missing derivation" to "failed to obtain derivation of"
since the path (e.g. '...drv^out') is a derivation output, not a derivation.
The message could be improved further to resolve ambiguity when multiple
outputOf links are involved, but for now we err on the side of brevity
since this message is already merged into larger error messages with
other context from the Worker and CLI.
(cherry picked from commit 3c3ceb18e9)
2ec62f1974