Changes the behavior of the "edit" command in GHCi.

Currently, when the user invokes the GHCi editor without a parameter
GHCi chooses the last file that loaded _sucessfully_.
This patch changes this behavior to start the editor with the
last file that _faild_ (this being what I usually want to edit).
Caveat: we use the module graph to determine what happened
and so we do not catch errors that occured during dependecy
analysis (e.g., a mis-typed module name) but we could fix this later.