Possibly-misleading error when forgetting the `number` argument to Resource `__init__`.

[cr1901]

Consider the following, snippet:

from nmigen import *
from nmigen_boards.tinyfpga_ax2 import *
from nmigen.build import *

plat = TinyFPGAAX2Platform()
plat.add_resources([Resource("led", Pins("1", dir="o", conn=("gpio", 0)))])

The above errors with:

$ python mk_build.py
Traceback (most recent call last):
  File "mk_build.py", line 8, in <module>
    plat.add_resources([Resource("led", Pins("gpio:13"))])
  File "C:/msys64/home/william/projects/fpga/nmigen/nmigen/nmigen/build/dsl.py", line 200, in __init__
    super().__init__(name, *args)
  File "C:/msys64/home/william/projects/fpga/nmigen/nmigen/nmigen/build/dsl.py", line 132, in __init__
    raise ValueError("Missing I/O constraints")
ValueError: Missing I/O constraints

Strictly-speaking, I don't think this is a missing I/O constraints problem. It's a "I forgot the number argument to Resource.__init__()" problem. It took me a few seconds to realize my mistake; I can see this error being frustrating to people who are trying to create resources for the first time. The correct code is: plat.add_resources([Resource("led", 0, Pins("1", dir="o", conn=("gpio", 0)))])- I didn't add any I/O constraints :).

I'd create a PR, but it's simply easier for me to open an issue right now :(.