mutable repair: return successful=False when numshares<k (thus repair fails),

mutable repair: return successful=False when numshares<k (thus repair fails),
instead of weird errors. Closes #874 and #786.

Previously, if the file had 0 shares, this would raise TypeError as it tried
to call download_version(None). If the file had some shares but fewer than
'k', it would incorrectly raise MustForceRepairError.

Added get_successful() to the IRepairResults API, to give repair() a place to
report non-code-bug problems like this.