Failure to specify a default to a wire no longer gives meaningful error message

[sdtwigg]

Previously, failure to specify the default for a wire (when using whens and similar) would give an error message indicate, at the very least, the name of the offending wire. Now, a NullPointerException is thrown in the width inference phase. The stack trace gives no meaningful info to the user.

[donggyukim]

Did you try it with the latest master? It should tell you problematic wires. Please refer to #339