No longer show __infer in pretty-printed errors

__infer is an implementation detail and shouldn't leak. Fixes #1626.

Old error:

    > a
    (input):1:1:When elaborating an application of constructor __infer:
            No such variable a

New error:

    > a
    (input):1:1:No such variable a
Merge pull request #1661 from david-christiansen/issue/1626

No longer show __infer in pretty-printed errors