Replace "throwError" with "flashMessage" and type

Currently, ``throwError`` is used for all manner of messages, including
errors, "success" messages, and "info" messages.  This makes appropriate
styling of the error message difficult.  In addition, the name
``throwError`` seems to create confusion, implying that an error will
actually be thrown (e.g. stopping execution when a user isn't logged in
[0][1]), when in fact it just displays a message.

This PR replaces ``throwError`` with ``flashMessage``, and reliably includes a
message "type" (e.g. "error", "success", "info") every time.  It renames
``lib/errors.js`` to ``lib/messages.js`` to more accurately reflect its
function.

This PR doesn't rename the message collection (``Errors``), nor the
template responsible for rendering the messages (``error_item.html``) --
that should probably still be done, but has higher likelihood of
trouble for existing alternate themes and installations.

[0] https://github.com/TelescopeJS/Telescope/blob/6ccf7d7d4704d6a8e821fe48128f81c19983ffc9/client/views/users/user_edit.js#L43
[1] https://github.com/TelescopeJS/Telescope/blob/083a4c4dc48eca15fe9d4472e24e6b4e8adfc8d6/client/views/users/user_email.js#L13