http: Return error if seek was not successful

Some webdav servers doesn't implement range requests, which are
necessary for a seek support. GVfsHttpInputStream doesn't handle this
case. Consequently g_seekable_tell returns a requested offset, however
g_input_stream_read returns bytes from a beginning of a file.

Return error if Content-Range header field is missing, or the range is
not valid. This change is needed to avoid data corruption when reading.

https://bugzilla.gnome.org/show_bug.cgi?id=754824