Less details in Solr connection error message

[FuhuXia]

Fixes #
Use a generic error message, not connection details in Solr error message.

Proposed fixes:

Features:

• includes tests covering changes
• includes updated documentation
• includes user-visible changes
• includes API changes
• includes bugfix for possible backport

Please [X] all the boxes above that apply

[amercader]

Can we get the details logged with a log.warning(e.args[0])? This might help maintainers debug the actual issue

[FuhuXia]

Changes were made so that
Site Visitors sees "Solr returned an error while searching.",
Site Maintainers sees "Connection Error: Failed to connect to Solr server."

[amercader]

There's another issue with the code merged in #8094 . The SolrConnectionError exception class for some reasons inherits from urllib3's NewConntectionError. This expects a connection as a first parameter, not a string.

This is causing typing errors and will probably cause other problems when we actually raise this exception. I don't see why we need to extend urllib3 class and we can't simply do the following like the rest of the search exceptions:

class SolrConnectionError(Exception):
    pass

@FuhuXia do you mind updating the exception class as part of this PR and check that the behaviour of #8094 is kept? Thanks