Improve error messages. (Bug#14808)

Improve error messages.  (Bug#14808)

* net/tramp.el (tramp-current-connection): New defvar, moved from
tramp-sh.el.
(tramp-message-show-progress-reporter-message): Removed, not
needed anymore.
(tramp-error-with-buffer): Show message in minibuffer.  Discard
input before waiting.  Reset connection timestamp.
(with-tramp-progress-reporter): Improve messages.
(tramp-process-actions): Use progress reporter.  Delete process in
case of error.  Improve messages.

* net/tramp-sh.el (tramp-barf-if-no-shell-prompt): Use
condition-case.  Call `tramp-error-with-buffer' with vector and buffer.
(tramp-current-connection): Removed.
(tramp-maybe-open-connection): The car of
`tramp-current-connection' are the first 3 slots of the vector.