Clarify warning for process-dependent input in ERC

Clarify warning for process-dependent input in ERC

* lisp/erc/erc.el (erc--check-prompt-input-for-running-process):
Resolve dissonance between content of ancient `user-error' message and
condition that triggered it by favoring the former because it's
supported by the underlying mechanism, which revolves around the
`process-not-needed' symbol property.
* test/lisp/erc/erc-tests.el (erc--check-prompt-input-functions):
Revise expected output for error assertion.  (Bug#66073, originally
from bug#54536, and included for discussion in bug#67677)