parser: more specific error messages; issue 3300 (issue 8506043)

[dak]

https://codereview.appspot.com/8506043/diff/1/lily/parser.yy
File lily/parser.yy (right):

https://codereview.appspot.com/8506043/diff/1/lily/parser.yy#newcode2323
lily/parser.yy:2323: | STRING
This not fully equivalent, but I think that LYRIC_ELEMENT is not
expected to occur here, so it is likely a reasonable choice of yours not
to mention it here.  Ok.

https://codereview.appspot.com/8506043/diff/1/lily/parser.yy#newcode3118
lily/parser.yy:3118: lyric_element optional_notemode_duration
post_events {
Here is the problem I have with this change: for
\new Lyrics { hello, Dolly }
the error message will become quite less helpful.

I have no good idea how to distinguish these two error classes (this
one, and the one you are thinking of) reliably in the syntax, so it
seems that we should design an error message that covers both cases.

Apropos: do you have an example of where your refactoring leads to a
better error behavior (independent from the error message text itself)?

https://codereview.appspot.com/8506043/