OpenSearch: Error for unsupported formats and adding format=jsonfm

Previously it would just completely ignore the format parameter.
Making it always serve JSON no matter the value of the format
parameter.

Now it properly errors when doing (for example) format=txt or format=xml.

Also adding support for jsonfm.

Change-Id: Ia98f54f41f39006312fb49ecd718f0f161f27c37
Merge "OpenSearch: Error for unsupported formats and adding format=jsonfm"