LockSet: encapsulate acquire() in try-except

This patch adds a try/except area around most of the acquire() code (everything
after the intial condition checks). Since the except: clause contains just a
'raise' nothing really changes except the indentation of the code.

This is done in a separate commit to insulate and make clearer what the real
code changes done in the upcoming patch are.

Reviewed-by: imsnah