Add synonym functionality to guru.WhichErrs for quick documentation

[dlsniper]

Guru, the code introspection tool from the Go team has the following functionality:

The whicherrs query reports the set of possible constants, global variables, and
concrete types that may appear in a value of type error.  This information may be
useful when handling errors to ensure all the important cases have been dealt with.

Example: a whicherrs query on the result of a call to http.ListenAndServe reveals
that the possible errors are EINVAL (a programmer error), a system call error, or a
network error.

https://docs.google.com/document/d/1_Y9xCEMj5S-7rv2ooHpZNH15JgRT5iM742gJkw5LtmQ/edit#heading=h.fy71cgl3pqh2