Improve error handling in Tramp

Improve error handling in Tramp

* lisp/net/tramp-compat.el (ls-lisp): Require.

* lisp/net/tramp.el (ls-lisp-use-insert-directory-program):
Don't declare.
(tramp-current-connection): Adapt docstring.
(tramp-debug-message): Adapt function names.
(tramp-error, tramp-run-real-handler): Let-bind `signal-hook-function'.
(tramp-signal-hook-function): New defun.
(tramp-debug-on-error, tramp-condition-case-unless-debug): Remove.
(tramp-file-name-handler): Handle `tramp-current-connection'.
Let-bind `signal-hook-function'.  Use `unwind-protect' instead of
`tramp-condition-case-unless-debug'.
(tramp-handle-insert-directory): Don't require ls-lisp.
(tramp-process-actions): Check, that
`tramp-password-save-function' is non-nil.
(tramp-equal-remote): Handle the case both files are local.

* test/lisp/net/tramp-tests.el (tramp--test-instrument-test-case):
Do not bind `tramp-debug-on-error'.
(tramp--test-ignore-make-symbolic-link-error): Make error handler
more explicit about the error.