Say which interfaces a multi-homed BMC has, rather than crash

Dedicated plus shared is how BMCs are built, so several NICs is ordinary.
_get_bmc_nic_url only reaches the count when the address the session came in
on matches none of them, which is what a tunnel or a NAT does, and it then
raised the bare PyghmiException base class.

Confluent has no handler for the base class, so it fell through to the
generic one and showed "Unexpected Error" plus a traceback, for a machine
doing nothing wrong. UnsupportedFunctionality now, which the redfish plugin
already reports plainly and which stays inside PyghmiException.

The message said "does not have exactly one interface" without saying how
many, which ones, or what to do. Every caller takes a name, so it lists the
candidates, and the empty case reads differently from the ambiguous one.

confluent_server/aiohmi/redfish/command.py

@@ -821,8 +821,7 @@ class Command(object):
         bmcinfo = await self._do_web_request(await self.get_bmcurl())
         nicurl = bmcinfo.get('EthernetInterfaces', {}).get('@odata.id', None)
         niclist = await self._do_web_request(nicurl)
-        foundnics = 0
-        lastnicurl = None
+        candidates = []
         oem = await self.oem()
         for nic in niclist.get('Members', []):
             curl = nic.get('@odata.id', None)
@@ -852,13 +851,26 @@ class Command(object):
                     socket.AF_INET6, addrs.get('Address', '::'))
                 if self._bmcv6ip == v6addr:
                     return curl
-            foundnics += 1
-            lastnicurl = curl
-        if name is None and foundnics != 1:
-            raise exc.PyghmiException(
-                'BMC does not have exactly one interface')
-        if name is None:
-            return lastnicurl
+            candidates.append(curl)
+        if name is not None:
+            return None
+        if len(candidates) == 1:
+            return candidates[0]
+        # UnsupportedFunctionality, not the bare base class, which confluent
+        # maps through its generic handler and shows as "Unexpected Error".
+        if not candidates:
+            raise exc.UnsupportedFunctionality(
+                'BMC published no enabled network interface of its own')
+        # Several NICs is ordinary. Reaching here means the address this
+        # session came in on matched none of them, which a tunnel or a NAT
+        # does, and picking one to reconfigure would be a guess. Every caller
+        # takes a name.
+        raise exc.UnsupportedFunctionality(
+            'BMC has {0} enabled interfaces and none of them carries the '
+            'address this session connected to, so which one is meant cannot '
+            'be determined. Name one of: {1}'.format(
+                len(candidates),
+                ', '.join(url.rsplit('/', 1)[-1] for url in candidates)))
 
     async def _bmcresetinfo(self):
         if not self._varresetbmcurl: