Add tests to check error handling of Graphite Web functions

[carrieedwards]

Is your feature request related to a problem? Please describe.
Currently, very few of the the supported Graphite web functions have tests that check for errors, such as passing in invalid parameters, are handled. We have found several cases where panics were occurring due to invalid parameters being passed. To prevent this and improve test coverage, error handling testing should be added to functions that currently do not have it.

Describe the solution you'd like
Every Graphite web function should have tests that verify error handling.