MAM: Return error when unsupported search fields are used

[guusdk]

Some clients use a data form field that is unrecognized. Up until now, the monitoring plugin has ignored those fields. This can lead to unexpected results, as the client (and end user) assume that a certain limitation is applied to the search, while in fact, it is not.

The plugin should return an error when unsupported fields are used, instead of silently ignoring them.