settings_ui: Revert extension for success callback in do_settings_change.
settings_ui: Revert extension for success callback in do_settings_change.

This reverts success callback extension for `do_settings_change` function
because it seems it is better to make the request directly rather
than calling `do_settings_change`.
And hence `error` callback extension is also removed  for the same
reason, but a error_continuation is added to do additional tasks when
errors happened.

shubhamdhama/zulip,synicalsyntax/zulip,tommyip/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,rishig/zulip,eeshangarg/zulip,timabbott/zulip,jackrzhang/zulip,zulip/zulip,zulip/zulip,brainwane/zulip,tommyip/zulip,rishig/zulip,zulip/zulip,tommyip/zulip,synicalsyntax/zulip,timabbott/zulip,rht/zulip,rht/zulip,kou/zulip,zulip/zulip,jackrzhang/zulip,hackerkid/zulip,rht/zulip,hackerkid/zulip,dhcrzf/zulip,rht/zulip,hackerkid/zulip,dhcrzf/zulip,jackrzhang/zulip,kou/zulip,andersk/zulip,andersk/zulip,brainwane/zulip,synicalsyntax/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,tommyip/zulip,hackerkid/zulip,dhcrzf/zulip,brainwane/zulip,kou/zulip,timabbott/zulip,andersk/zulip,shubhamdhama/zulip,brainwane/zulip,punchagan/zulip,showell/zulip,dhcrzf/zulip,timabbott/zulip,timabbott/zulip,kou/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,tommyip/zulip,rishig/zulip,brainwane/zulip,synicalsyntax/zulip,zulip/zulip,showell/zulip,rht/zulip,kou/zulip,andersk/zulip,shubhamdhama/zulip,showell/zulip,brainwane/zulip,jackrzhang/zulip,rishig/zulip,tommyip/zulip,brainwane/zulip,shubhamdhama/zulip,shubhamdhama/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,jackrzhang/zulip,rishig/zulip,dhcrzf/zulip,andersk/zulip,punchagan/zulip,shubhamdhama/zulip,dhcrzf/zulip,andersk/zulip,rishig/zulip,eeshangarg/zulip,rht/zulip,shubhamdhama/zulip,showell/zulip,hackerkid/zulip,synicalsyntax/zulip,timabbott/zulip,punchagan/zulip,timabbott/zulip,synicalsyntax/zulip,synicalsyntax/zulip,kou/zulip,zulip/zulip,jackrzhang/zulip,showell/zulip,eeshangarg/zulip,dhcrzf/zulip,showell/zulip,tommyip/zulip,kou/zulip,punchagan/zulip,jackrzhang/zulip,rishig/zulip