MLCE-825: Give reason when workload unsupported for Non Constant Weights/Bias

 * BackendHelper.cpp IsXXXLayerSupported doesn't get as far as Neon/Cl
   Validate functions where arm_compute::Status is returned.
 * Conv2d, Depthwise, DilatedDepthwise and FullyConnected
 * Tidy up if() -> if ()
 * Clean up logic in FullyConnected so that isLayerSupported gets called

Signed-off-by: Francis Murtagh <francis.murtagh@arm.com>

Signed-off-by: Teresa Charlin <teresa.charlinreyes@arm.com>

Change-Id: I5da1a882f4a2f55e90aa984b2b9548a847cb3a2d