Fix for Bug 4946 - hold warning needs rewording

Fix for Bug 4946 - hold warning needs rewording

This patch improves the phrasing of several messages by
breaking the message variable into distinct parts for more
natural-sounding warnings when:

- Checking out an item on hold for another patron
- Checking out an item which is waiting for another patron
- Checking out an item which is checked out to another patron
- Checking out an item which is checked out to this patron
- Checking out to a patron who has too many checked out

I would appreciate special attention to my changes to the
TooMany function in Circulation.pm to make sure I handled
it correctly.

Signed-off-by: Nicole Engard <nengard@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>