Improve error handling in tramp-gvfs

Improve error handling in tramp-gvfs

* lisp/net/tramp-gvfs.el (tramp-gvfs-get-directory-attributes)
(tramp-gvfs-get-root-attributes)
(tramp-gvfs-handle-file-attributes): Don't ignore errors.
(tramp-make-goa-name): New defun.
(tramp-gvfs-get-remote-prefix): Use it.
(tramp-gvfs-maybe-open-connection): Raise user errors in case of.
Check also, that GOA accounts are proper.
(tramp-get-goa-accounts): Cache connection property.

* lisp/net/tramp.el (tramp-handle-file-equal-p)
(tramp-handle-file-in-directory-p): Use `tramp-equal-remote'.