topdown/http: Respect raise_error flag during input validation

Currently the `raise_error` flag is not honored during the
input validation step. So `http.send` will return an error if
input validation fails irrespective of the `raise_error` flag
status. This change attempts to fix that.
Also the description of the `raise_error` flag is updated to
reflect actual behavior.

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>