runtime: PanicNilError error string should have "runtime error: " prefix [1.21 backport]

[gopherbot]

@mauri870 requested issue #63813 to be considered for backport to the next 1.21 minor release.

@gopherbot please open a backport issue for Go 1.21.

Errors that implement runtime.Error should have a "runtime error: " prefix, with the solo exception of runtime.plainError on purpose. Calling panic(nil) results in a PanicNilError that violates this constraint.

[cherrymui]

Waiting for the decision of #63813