Replace some uses of erc-error

Replace some uses of erc-error

* lisp/erc/erc-button.el
(erc-button--display-error-notice-with-keys-and-warn): Use
`erc--lwarn' so the warnings buffer is overridable for testing.
* lisp/erc/erc-sasl.el (erc-sasl-mode, erc-sasl-enable): Signal an
`error' instead of calling `erc-error', which continues execution.  In
this special case, the session cannot continue initializing, since
connection registration can't reasonably be expected to complete
successfully.
(erc-sasl--destroy): Don't run `erc-quit-hook', and issue a warning of
level `:error' to get users' attention instead of calling `ding'.
* lisp/erc/erc-speedbar.el
(erc-speedbar--emulate-sidebar-set-window-preserve-size): Don't set
window parameters.  Doing this basically made
`erc-speedbar-toggle-nicknames-window-lock' unusable.
(erc-speedbar--toggle-nicknames-sidebar): Manually unlock the window
after toggling.
(erc-nickbar-mode, erc-nickbar-enable, erc-nickbar-disable): Don't
`ding' when called in a non-ERC buffer, and make sure to call
`erc-speedbar--ensure' from an ERC buffer.  Also, don't disable
minor-mode var when speedbar buffer doesn't exist because that doesn't
ensure it'll be created the next time around, and users may count on
the activation state remaining consistent.
(erc-speedbar-toggle-nicknames-window-lock): Make usable from lisp
with explicit numeric arg.
* lisp/erc/erc-status-sidebar.el (erc-bufbar-mode, erc-bufbar-enable):
Only create the side window from an erc-mode buffer to ensure the
ratio is preserved when burying the current buffer, e.g., with
`custom-buffer-done'.
* lisp/erc/erc.el (erc--warnings-buffer-name, erc--lwarn): New
function, an analog of `lwarn', that allows for overriding the
warnings buffer with the new variable `erc--warnings-buffer-name'.
(erc-cmd-SERVER): Add comment.
* test/lisp/erc/erc-scenarios-sasl.el
(erc-scenarios-sasl--plain-fail): Expect warning instead of error.
* test/lisp/erc/erc-scenarios-status-sidebar.el
(erc-scenarios-status-sidebar--bufbar): Refresh when interactive as
well.
* test/lisp/erc/resources/sasl/plain-failed.eld: Expect EOF instead of
"CAP END".  (Bug#63595)