#9 file and directory errors

[Mark Lentczner]

Errors from the underlying files and directories need to be
handled throughout the XML layers.

XMLReader and XMLWriter should handle errors in the file
by throwing exceptions.

XMLMedia needs to gracefully handle missing directories
and files. It isn't acceptable to refuse to mount some XML
tree because one file deep within it is missing. Similarly,
writing a tree must continue around some directory or file
that can't be written.

The errors in these cases should be logged, and if the user
fixes them, then XMLMedia should log the first success on
future read/writes.

[Mark Lentczner]

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note that all this applies to errors with XMLStorage objects with
missing or errors with streams and subcontainers