rdar://problem/11584012

Refactorings of watchpoint creation APIs so that SBTarget::WatchAddress(), SBValue::Watch(), and SBValue::WatchPointee()
now take an additional 'SBError &error' parameter (at the end) to contain the reason if there is some failure in the
operation.  Update 'watchpoint set variable/expression' commands to take advantage of that.

Update existing test cases to reflect the API change and add test cases to verify that the SBError mechanism works for
SBTarget::WatchAddress() by passing an invalid watch_size.

llvm-svn: 157964