Remove non-functional session load error
MatrixChat was trying to display an error if the session failed to
restore, but it was never actually being shown because it was just
set as a member variable and therefore never actually caused
a re-render for the error to be displayed. Almost all errors are
caught by _restoreFromLocalStorage which displays the fancy dialog
if your session can't be restored, so I'm not convinced this ever
even tried to do anything anyway. Remove it.
