[LibArchFPGA] Unknown attributes are not being warned / errored

[AlexandreSinger]

See #3403

There are three examples shown in this PR:

1. "num_pins" attribute in mode
2. "num_pins" attribute in interconnect tags
3. Missing "Cout" attribute in switch tag.

This attributes are all ignored / given default values; however, these should throw an error to prevent people writing syntax they do not intend.

[AlexandreSinger]

It looks like the Cout attribute can be left off specifically for shorts. This is not well documented and should be improved.