Provide a better error message when a user enters a Conduit parameter string without quotes around it (and similar)

Summary: See D1010. The API uniformly requires JSON, which is good for
strictness and predictablity but can be bad for UEX, especially considering that
we silently continue after failing to decode things. Toss the user a lifeline
when they make this common mistake.

Test Plan: Ran API calls with invalid and valid inputs. Invalid inputs gave me a
reasonable error message.

Reviewers: davidreuss, jungejason, nh, tuomaspelkonen, aran

Reviewed By: nh

CC: aran, nh

Differential Revision: 1012