Bad error message for duplicate ClockDomain

[rroohhh]

Adding a ClockDomain with the same name multiple times gives a bad error message. This example:

from nmigen import *
from nmigen.back.verilog import convert

m = Module()
m.domains += ClockDomain("a")
m.domains += ClockDomain("a")

convert(m)

produces this output:

  File "a.py", line 8, in <module>
    convert(m)
  File "/home/robin/.guix-profile/lib/python3.7/site-packages/nmigen/back/verilog.py", line 86, in convert
    rtlil_text = rtlil.convert(*args, **kwargs)
  File "/home/robin/.guix-profile/lib/python3.7/site-packages/nmigen/back/rtlil.py", line 1039, in convert
    fragment = ir.Fragment.get(elaboratable, platform).prepare(**kwargs)
  File "/home/robin/.guix-profile/lib/python3.7/site-packages/nmigen/hdl/ir.py", line 39, in get
    obj = obj.elaborate(platform)
  File "/home/robin/.guix-profile/lib/python3.7/site-packages/nmigen/hdl/dsl.py", line 544, in elaborate
    fragment.add_domains(self._domains)
  File "/home/robin/.guix-profile/lib/python3.7/site-packages/nmigen/hdl/ir.py", line 117, in add_domains
    assert domain.name not in self.domains
AssertionError