Improve error messages in ulex parsing

[Kaljurand]

When loading the user lexicon, e.g. by

./ape.exe -text "" -ulexfile mylex.pl

then some syntactically incorrect lexicon entries, e.g.

noun_sg(Human, Human, neutr).
noun_sg(human being, human being, neutr).

currently result in uninformative error messages, e.g.

value="Malformed entry."
repair="User lexicon entry is malformed: noun_sg(_G1234, _G1234, neutr)"

value="Malformed file."
repair="The user lexicon file is not a valid Prolog file."