Improve get_ip_interface error message when interface does not exist

[lesteve]

Context: in dask-jobqueue you need some tweaking to find the right network interface to use on your cluster. People may not be familiar with what a network interface is and the current error message is a bit cryptic.

The main thing I am wondering about is whether we should keep the same error type KeyError with backward-compatibility in mind or switch to ValueError since ValueError is already the type of the error you get when the network interface does not have a valid IPv4 address.

My personal preference would be to switch to ValueError but I have kept KeyError for now.

Error message on master:

In [1]: from distributed.utils import get_ip_interface 
   ...: get_ip_interface('asdf')                                                                                                                                                   
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-1-cf8f3124444e> in <module>
      1 from distributed.utils import get_ip_interface
----> 2 get_ip_interface('asdf')

/home/local/lesteve/dev/distributed/distributed/utils.py in get_ip_interface(ifname)
    170     import psutil
    171 
--> 172     for info in psutil.net_if_addrs()[ifname]:
    173         if info.family == socket.AF_INET:
    174             return info.address

KeyError: 'asdf'

Error message in this PR:

In [1]: from distributed.utils import get_ip_interface 
   ...: get_ip_interface('asdf')                                                                                                                                                   
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-1-cf8f3124444e> in <module>
      1 from distributed.utils import get_ip_interface
----> 2 get_ip_interface('asdf')

/home/local/lesteve/dev/distributed/distributed/utils.py in get_ip_interface(ifname)
    175         allowed_ifnames = list(net_if_addrs.keys())
    176         raise KeyError('{!r} is not a valid network interface. '
--> 177                          'Valid network interfaces are: {}'.format(ifname, allowed_ifnames))
    178 
    179     for info in net_if_addrs[ifname]:

KeyError: "'asdf' is not a valid network interface. Valid network interfaces are: ['lo', 'enp0s25', 'br-e1ab2121f792', 'docker0', 'br-9f15079dee0d']"

[mrocklin]

Small nit:

Rather than test that the entire error message is exactly this, we might instead test some properties of it that we care about, like that "network interface" and the names of other interfaces are present in the message.

If someone changes the error message but includes this kind of information I think that that's ok, and shouldn't be considered an error.

[mrocklin]

This is great. I really like changes like this!

[mrocklin]

Thanks for this change @lesteve ! I left one small nit, but I'm totally fine with this PR as-is.

I'm fine with changing the error type if you prefer.

[lesteve]

I pushed a commit:

• the type of the error is now ValueError
• I reduced the test dependence on the specific wording of the error message. Now I am only testing that ifname, "network interface" and "lo" (or its OSX equivalent) are all in the error message (through a regex)

[mrocklin]

Thanks @lesteve !