Suppress an error on a sentinel.

* twittering-mode.el: Suppress an error on a sentinel. Previously,
an error might be raised from a sentinel when a timeline of
non-existent user was requested. A sentinel for such a request
tries to remove the timeline spec string corresponding to
non-existent user from the history. In the process, conversion
from an invalid timeline spec string had caused an error.
(twittering-string-to-timeline-spec): add an optional argument
NOERROR. If NOERROR is non-nil, the function does not raise an
error even if SPEC-STR is invalid as a timeline spec string.
(twittering-equal-string-as-timeline): do not raise an error even
if either SPEC-STR1 or SPEC-STR2 is invalid as a timeline spec
string.
Reduce API rate limit information on mode-line.

* twittering-mode.el: Reduce API rate limit information on
mode-line.
(twittering-get-ratelimit-indicator-string): reduce timelines
retrieved by a common API.