Compose parser is too lenient

[wismill]

The following file parses correctly, although no sequence is valid!

! include ~
! include x
! include ""
include
include !
include x
# include ""
:
a :
: a
<a> : a a
!:
<a> : !

We should fail at least on the includes.

Found while looking for a Compose compile failure in #360.

[bluetech]

There is a max of 10 errors:

libxkbcommon/src/compose/parser.c

Line 575 in 502e9e5

enum { MAX_ERRORS = 10 };

If I understand, you're thinking

1. If no valid sequences at all at the end, error.
2. If include fails, fail fast.

(1) sounds sensible to me. (2) might be a backward compat issue though. I can imagine people having some bad includes which they ignore but now the file will stop loading. But maybe it's OK to break?

[wismill]

The are two issues with includes here:

• bad syntax: it should fail early. We may be lenient for some other syntax issues, but I think we should not for includes.
• file not found: skip and emit an error