ENHANCED: http_parameters/2,3: error handling.  Errors from http_parameters
(missing parameter, wrong type) are now mapped to "400 Bad Request", the message
is more informative and also if library(http/http_error) is loaded, the stack
trace is omitted.  After all, these are not server errors.

MODIFIED: Now uses existence_error(http_parameter, Name) rather than
existence_error(form_data, Name).
Updated header
ENHANCED: Raise an error when trying to use http_parameters for the second
time on POST data.
DOC: http_parameters/2,3 updated.
ENHANCED: More human friendly error messages from http_parameters/2,3.