<rdar://problem/10017623>

Fixed an error where if we tried to format a ValueObject using a format
that was incorrect for a variable, then it would set ValueObject::m_error
to an error state and stop the value from being able to be updated. We now
leave m_error alone and only let the update value code change that. Any errors
in formatting will return a valid value as C string that contains an error 
string. This lets us then modify the format and redisplay without any issues.

llvm-svn: 151581