Give better error message if a helpfile is missing

[fingolfin]

A recent xgap release had a bug: it's old-style manual was missing a file (gobject.tex) in the release tarball (see gap-packages/xgap#3). This lead to a rather unhelpful error message by the GAP help system:

Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `PositionStream' on 1 arguments called from
PositionStream( stream 
 ) at /mnt/disk2/hudson-slave/workspace/GAP-update-release-test/GAPCOPTS/64bui\
ld/GAPGMP/gmp/GAPTARGET/standard/label/64bit/gap4r8/lib/helpdef.gi:306 called \
from
...

That was the error @alex-konovalov got, I actually saw a different one:

Error, Function Calls: <func> must return a value in
  return HELP_PRINT_SECTION_TEXT( info, chnr, secnr ); at /home/r/mhorn/gap/lib/helpdef.gi:669 called from
HELP_BOOK_HANDLER.(book.handler).HelpData( book, entrynr, type ) at /home/r/mhorn/gap/lib/helpbase.gi:783 called from
...

This PR improves on the situation, by inserting explicit error messages that at least give a hint as to what went wrong, making it easier to debug similar issues in the future.

I also fixed HELP_PRINT_SECTION_TEXT to always return a value (see also issue #464 and PR #466).

[codecov-io]

Current coverage is 48.82% (diff: 0.00%)

Merging #939 into master will decrease coverage by <.01%

@@             master       #939   diff @@
==========================================
  Files           424        424          
  Lines        222089     222094     +5   
  Methods        3426       3426          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits         108441     108436     -5   
- Misses       113648     113658    +10   
  Partials          0          0          

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[ChrisJefferson]

Both changes here look good (and unfortunately tricky to test, so lack of tests is OK I think).

[olexandr-konovalov]

The different error messages are fine: I have reported what I have seen in the stable-4.8 branch while testing the release candidate for the next minor release, while tests that were performed in the master branch failed in the same way as @fingolfin observed:

########> Diff in /home/hudson/hudson/workspace/GAP-pkg-update-master-quicktes\
t/GAPCOPTS/64build/GAPGMP/gmp/GAPTARGET/standard/label/graupius/GAP-pkg-update\
-master-snapshot/tst/teststandard/helpsys.tst:24
# Input is:
for i in [1..Length(HELP_LAST.TOPICS)] do HELP(String(i)); od;
# Expected output:
# But found:
Error, Function Calls: <func> must return a value
########

The PR looks good to me, and it addresses both issues. I will merge it after GAP 4.8.6 will be out, to not to interfere with the release workflow.

[fingolfin]

@alex-konovalov This PR is against master, not stable. I did not think it was worth the bother. So I don't see how merging this could interfere with the release workflow... ?

BTW, this is one of those PRs where I'd suggest to rebase, not merge (which the github API nicely allows these days).

[olexandr-konovalov]

@fingolfin ah, you're right - this is master branch of course. I will rebase it now - thanks.