Add new macro `ignore-error'

* doc/lispref/control.texi (Handling Errors): Document
`ignore-error'.
* lisp/subr.el (ignore-error): New macro.

* lisp/progmodes/elisp-mode.el (elisp-completion-at-point):
Provide completion for `ignore-error'.
1 job for master in 56 minutes and 20 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #2705
test-all

00:56:20

 
Name Stage Failure
failed
test-all Test
  lib-src/emacsclient-tests.log
lisp/net/tramp-tests.log
make[2]: *** [check-doit] Error 1
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1