Fix error message in replace-match for subexpressions

* src/search.c (Freplace_match): Output a more understandable
error message when replacing a subexpression (bug#19208).
1 job for master in 56 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #2867
test-all

00:56:17

 
Name Stage Failure
failed
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1