Non-channel argument in `config` of top-level proc gives very unhelpful error message

[allight]

Describe the bug
The top proc of a dslx design must only contain 'channel' arguments to be convertable into IR. If code is given to ir_converter_main which does not match this requirement a highly confusing error message saying NOT_FOUND: Top-level type info not found for proc is emitted instead of a message that gives any information about what went wrong.

To Reproduce
Steps to reproduce the behavior:

1. run ir_converter_main on the following DSLX code:

proc datasource {
  off: u32;
  o: chan<u32> out;
  config (o: chan<u32> out, val: u32) { (val, o) }
  init {()}
  next(state: ()) {
    send(join(), o, off);
  }
}

Expected behavior
An error saying something to the effect of 'val' is not constant expression.

Current behavior

Error: NOT_FOUND: Top-level type info not found for proc "datasource".

Additional info
This was brought to our attention by #1663

[allight]

@richmckeever PTAL.