In geocoder, add support for error messages in json

[mraross]

Currently, all error responses are in html format.

Change error handling so that:

1. A request for HTML or XHTML output returns an error in HTML format (as currently supported).
2. A request for KML output returns an error in KML format (as currently supported).
3. A request for json, geojson, CSV, or ShapeFile output returns an error in JSON format.

This is needed by JSON developers since JSON is the format they expect error objects to be in.

I propose we adopt the Google json error object as defined in:

https://google.github.io/styleguide/jsoncstyleguide.xml?showone=error#error
https://google.github.io/styleguide/jsoncstyleguide.xml?showone=error#error

So, for example, a request for a non-existent siteID and json output will return an HTTP Response code of 404 and the following JSON error object:

{
"apiVersion": "3.4",
"error": {
"code": 404,
"message": "Site Not Found",
}
}

[BK01]

Return a 404 for hard errors. For 'site not found' errors, return a 200 (see #333).

[BK01]

Validated in geocodertst (version 4.3)