Control.Monad.Error documentation

Documentation for the Error monad. Please take a minute to review it. This is one those parts of the library newbies have difficulties to grok.

Converted the module documentation to Haddock format. Per Jeff Newbern's gracious permission included relevant informationan example from his cool tutorial "All About Monads" http://www.nomaware.com/monads/. Added an example for ErrorT. Use String instead of [Char].

Deadline for the proposal - 2 weeks.

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries (other)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Multiple
Architecture	Multiple