<rdar://problem/12586188> Make ImportError a special case for "command script...

<rdar://problem/12586188> Make ImportError a special case for "command script import", such that the error message for the exception becomes the error for the entire import operation
and silence the backtrace printout

In the process, refactor the Execute* commands in ScriptInterpreter to take an options object, and add a new setting to not mask out errors so that the callers can handle them directly
instead of having the default behavior

llvm-svn: 167067