Structural Navigation: Handle unimplemented collection interface error

The collection interface is something applications and toolkits get
"for free" from AT-SPI2. However, applications/toolkits that provide
their own implementation of AT-SPI2 (e.g. WebKitGtk) presumably need
to also implement AtspiCollection themselves, or do something so that
the default AT-SPI2 implementation is used. Currently, in WebKitGtk
version 2.36, a non-implemented error occurs when querying the
collection interface on the document. We were not handling this error,
causing Orca to be silent when structural navigation commands were
used. This commit handles the error. It cannot solve the missing
implementation, however. Thus in WebKitGtk apps,  Orca will now announce
things like "no more headings" when there are in fact headings present.

See issue #244