Ignore non-existing attr in APIResourceWrapper __repr__

Fixes bug 1202415

__repr__ was added to APIResourceWrapper in the recent commit,
but it searches all attributes in _attrs list. When an attribute
defined in _attrs actually does not exist, __repr__() fails.
This commit changes __repr__ to ignore non-existing attributes.

Change-Id: Iebaeae78f3763d87f3993ba5c4bbed4c23e84c45
Merge "Ignore non-existing attr in APIResourceWrapper __repr__"