engraver: continue when trying to create non-existent Grob (issue 547620043 by address@hidden)

[dak]

https://codereview.appspot.com/547620043/diff/545560043/lily/engraver.cc
File lily/engraver.cc (right):

https://codereview.appspot.com/547620043/diff/545560043/lily/engraver.cc#newcode126
lily/engraver.cc:126: grob = new Item (SCM_EOL);
Ugh.  How is that going to work?  I am not sure we aren't better off
with a hard exit.  Admittedly, a failed assertion makes little sense
after a programming_error: this should have been a hard error in the
first place.

What is the use case?

https://codereview.appspot.com/547620043/