Prevent a weird search error when displaying some facet

[noirbizarre]

This PR fix a weird error only happening on some config/servers when displaying search result.
I wasn't able to reproduce the issue on my own computer but I was able to identify and fix the issue on a server where it's happening.

The Bad Request error page is displayed because of this https://github.com/opendatateam/udata/blob/master/udata/frontend/error_handlers.py#L9-L11
This make sense because most of the framework (WTForm, Flask-Principal, Flask-RESTPlus...) permit to use ValueError as a validation error (and that's what is done everywhere in udata). The problem is that UnicodeError and child (UnicodeDecodeError, UnicodeEncodeError...) extends ValueError.

To fix this issue (in another PR), there is 2 possibilities:

• use more specific exceptions that ValueError to handle this (prefered)
• make a special case for UnicodeError and child (hackish)