0005874: Improve error message for polymorphic variant

Notes
(0008700) gasche (developer) 2013-01-06 15:41	Regarding typos, I think we could do something along the following lines: when there is an unification mismatch between two variant types, check the labels that appear in one but not the other with the typo-finding algorith, using the set of labels on the other side as dictionary. Maybe this should be restricted to annotation propagation, deciding to trust the annotation rather than the term.

(0008701) hongboz (developer) 2013-01-06 18:33	I agree, the type annotation should help to give a more friendly hint. And it may be helpful to offer two errors mode, one is abbreviate which provides most useful content, another is verbose mode. Sometimes the error message I got is like 20 pages long(two module types unification mismatch)..

(0009266) hongboz (developer) 2013-05-10 16:47	In some cases, when type a mis-match type b, the compiler simply dumps the two types. Suppose type a is big, the error message is ridiculously large. Some simple analysis would help a lot, suppose a is a concrete type while b is a function type, the compiler could emit a more friendly error message(see tmp.ml)

(0009268) hongboz (developer) 2013-05-10 16:50	Other analysis would help suppose type a = big_type list while type b = big_type array when type a mis-matches type b, the compiler only need tell me one is list the other is array instead of dumping the two types

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