Put exceptionHandling for isReadable() / isWriteable() into the TransferElement base class

[killenb]

For postRead() and postWrite() the call to _exceptionBackend::setException() is happening in the TransferElement base class.
isReadable() and isWriteable() also have to call setException() when they raise a ChimeraTK::runtime_error. Currently all implementations have to do this themselves.

Taks:

• Move all existing isReadable() and isWriteable() implementations to isReadable_impl() and isWriteable_impl() in all backends
• Introduce non-virtual functions isReadable() and isWriteable() which
  • catch runtime_errors throw by the _impl()
  • call _exceptionBackend::setException()
  • re-throw the exception
• Make isReadOnly() a fixed, non-virtual implementation which returns isReadable() && !isWriteable(), and remove all implementations in the backends

As this is an interface change which requires to touch all backends, it should be done together with #165, which also requires that.