lib: rename ErrorMayQuit to ErrorNoReturn

[fingolfin]

See issue #470

Note that if this merged, it should be merged both into stable-4.8 and master. And in master, one additional change will be needed, as it also uses ErrorMayQuit in lib/files.gi:46

[olexandr-konovalov]

Before merging into stable-4.8, just to make sure that @ChrisJefferson and @james-d-mitchell are aware of our request to update your profiling and semigroups packages - could you manage this before 1st Feb 2016, please?

[fingolfin]

Perhaps I should not completely remove ErrorMayQuit immediately, instead turning it into a synonym -- and then remove it once profiling and semigroups are adapted. Otherwise, we get a chicken-egg-dependency problem...

[fingolfin]

I just did that. We could now merge this, then coax @james-d-mitchell and @ChrisJefferson to adapt their packages; once they did it, we can remove the alias.