FCPInterface: Specify deprecation mechanism for fields of the
SimpleFieldSets which WOT sends

- The plugin-fcp-rewrite of fred will have special standardized
errorCode and errorMessage values natively at fred plugin FCP messages.
Thus, the following commits which adapt the
FCPInterface.errorMessageFCP() will have to deprecate the old error code
and message fields which were previously put  into the SimpleFieldSet of
the message in a non-standardized way. So it is a good time to
standardize a deprecation mechanism for SFS entries of WOT.
- I chose to include the deprecated field list in the actual on-network
data to ensure that client authors read them :) Also, it will make it
easier to understand dumps of raw FCP traffic. For example during the
RFC of event-notifications, I had supplied a dump to the mailing list.
But as it contained lot of deprecated fields, people got confused and
had to ask about the duplicate fields and which ones are the right ones
etc.
FCPInterface.errorMessageFCP(): Fix to work with new errorCode /
errorMessage fields of fred branch plugin-fcp-rewrite

- Have two copies of the function:
* One for eating generic, unkown Exceptions. They will be specified as
errorCode = "InternalError" as requested by the plugin-fcp-rewrite API.
* The other for specific, well-known exceptions such as "there is no
Identity with the ID you specified" which can have a proper errorCode
which is parsable by the client.
- Tell the user to prefer the one with the proper error code in the
JavaDoc.
- Also add a FIXME at the function which uses the errorMessageFCP() the
most to specify proper error codes.
- In conclusion, deprecate the "Error" and "Description" custom,
non-standard WOT fields and add a TODO to remove them after some time
- Notice that the value of OriginalMessage is correctly set as it was
previously passed by the caller which was changed to use the new
function signature at c9b524c69128fcc826207d2a67b05f76ecbbac2e: The diff
of that commit shows the value