Extra hint in "No Method Found" if one of the arguments is 'fail'

[olexandr-konovalov]

This PR is an alternative to PR #355 by @ChrisJefferson.

It originates from issue #354 initiated by @cdwensley, and adds an extra hint in the "No Method Found" message in the case where one of the arguments is fail, as suggested by @stevelinton.

For example:

gap> Read(Filename(DirectoriesLibrary("tst"),"testall.g"));
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
The 1st argument is 'fail' which might point to an earlier problem
Error, no 1st choice method found for `Read' on 1 arguments called from
...

[olexandr-konovalov]

Can we get this reviewed an merged? I could then cherry-pick it onto stable-4.8.

[olexandr-konovalov]

@stevelinton @fingolfin does the new error message

gap> Read(Filename(DirectoriesLibrary("tst"),"testall.g"));
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
The 1st argument is 'fail' which might point to an earlier problem
Error, no 1st choice method found for `Read' on 1 arguments called from
...

look OK to you? In this case, this is ready to be merged.

[fingolfin]

I am a bit torn on this one. On the one hand, it seems strange to have a special case for this particular thing. On the other hand, it seems like a pragmatic solution to a common problem.

Hmm, thinking about this, I can sum it up like this: My ratio tells me this is not pure, and thus "bad", my guts tell me "screw purity, Alex put work into this, it helps with a real world problem, it hurts nobody, and if somebody has a better idea, they can provide their own PR with that, replacing this solution".

So, I'll just merge it for now.