(bug 49498) Handle errors from invalid snak values.

When providing invalid snak values to the API, we should respond
with a meaningful error message, not a fatal error.

Note that this needs I4d6b27a to be applied to the DataValues
project in order to work correctly.

Change-Id: I09e54dca33a3d143b1fa0d2f6ccf331ba8241a36