check_node(): stop spraying mystery output to stderr.

check_node():  stop spraying mystery output to stderr.

When a node number disagrees, keep track of all sources & the
node numbers they reported, and stick all that in the error message.

Changed all callers to supply a non-empty "source" argument; made
the "source" argument non-optional.

On my box, test_uuid still fails, but with the less confusing output:

AssertionError: different sources disagree on node:
    from source 'getnode1', node was 00038a000015
    from source 'getnode2', node was 00038a000015
    from source 'ipconfig', node was 001111b2b7bf

Only the last one appears to be correct; e.g.,

C:\Code\python\PCbuild>getmac

Physical Address    Transport Name
=================== ==========================================================
00-11-11-B2-B7-BF   \Device\Tcpip_{190FB163-5AFD-4483-86A1-2FE16AC61FF1}
62-A1-AC-6C-FD-BE   \Device\Tcpip_{8F77DF5A-EA3D-4F1D-975E-D472CEE6438A}
E2-1F-01-C6-5D-88   \Device\Tcpip_{CD18F76B-2EF3-409F-9B8A-6481EE70A1E4}

I can't find anything on my box with MAC 00-03-8a-00-00-15, and am
not clear on where that comes from.