[errors] Add functions to Wrap errors and preserve type

[jeromefroe]

This PR adds two functions, Wrap and Wrapf, that allow one to wrap an error with additional context while still preserving the underlying type of the error. These functions address the problem that one cannot use fmt.Errorf to wrap errors because that loses the type information of the error which is being wrapped. One can accomplish the same thing currently with NewRenamedError and Wrap and Wrapf aim to make such tasks easier to perform.

[coveralls]

Coverage increased (+3.1%) to 79.122% when pulling 98ceb19 on jeromefroe/errors/add-functions-to-wrap-errors into 3f78c00 on master.

[prateek]

LGTM