Prevent borrower import from overwriting data that it shouldn't, and polish off a...

Prevent borrower import from overwriting data that it shouldn't, and polish off a couple error conditions.

When an imported patron matches an existing one, keep data in the original record unless our import file
includes that column.  Also, check for branch & categorycodes, and produce error if invalid.  Finally,
auto-calculate expiry date if not given.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>