[Feature] Inform the user when they supply just the weights in place of the full model

[IgorTatarnikov]

Currently, if a user provides just the weights in place of the full model to cellfinder they'll get the following error:

Input 0 of layer "resunit2_block0_bn_a" is incompatible with the layer: expected axis 3 of input shape to have value 6, but received input with shape (64, 12, 13, 7, 64)

The cause and solution should be better communicated to the end user via an error message.