Indent terms printed in typechecking errors

This should make errors more readable, i.e. instead of printing:

    Can't unify <SOME_LONG_TYPE> with <SOME_OTHER_VERY_LONG_TYPE>

this change makes it print

    Can't unify
        <SOME_LONG_TYPE>
    with
        <SOME_OTHER_LONG_TYPE>

and similarly for other errors.
Merge pull request #483 from yfyf/indent_errors

Indent terms printed in typechecking errors