Give better error messages in image-convert

* lisp/image/image-converter.el (image-convert): Make
image-convert bug out earlier on a wrong IMAGE-FORMAT value
(bug#38310).
1 job for master in 62 minutes and 47 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #4318
test-all

01:02:47

 
Name Stage Failure
failed
test-all Test
make[2]: *** [check-doit] Error 1
make[1]: *** [check-expensive] Error 2
make: *** [check-expensive] Error 2
Makefile:319: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1