Fix ZFS feature generation script

This was breaking on FreeBSD, as some of the URLs were returning 404.  That
raises an HTTPError.  If we catch those and skip those sources (just as we
were already doing for HTTP codes other than 200), the script completes.  But
then we get a whole bunch of columns full of "no", which does not seem useful.
I added code to skip columns for sources that failed to download.

Signed-off-by: Richard Laager <rlaager@wiktel.com>

scripts/compatibility_matrix.py

@@ -8,6 +8,7 @@ import logging
 import sys
 
 from collections import defaultdict
+from urllib.error import HTTPError
 from urllib.request import urlopen
 from datetime import datetime
 from re import sub as regex, findall
@@ -160,13 +161,20 @@ features = defaultdict(list)
 readonly = dict()
 
 for name, sub in sources.items():
+    found = {}
     LOG.debug('Work on %s...', name)
     for ver, url in sub.items():
         LOG.debug('Get %s...', url)
-        with urlopen(url) as c:
+        try:
-            if c.getcode() != 200:
+            with urlopen(url) as c:
-                continue
+                if c.getcode() != 200:
-            man = c.read().decode('utf-8')
+                    LOG.debug('Failed with HTTP code %d', c.getcode())
+                    continue
+                man = c.read().decode('utf-8')
+        except HTTPError:
+            LOG.debug('Failed with HTTPError')
+            continue
+        found[ver] = url
         for line in man.split('\n'):
             if line.startswith('.It '):
                 line = line[4:]
@@ -191,6 +199,7 @@ for name, sub in sources.items():
         # https://github.com/Nexenta/illumos-nexenta/blob/release-4.0.4-FP/usr/src/common/zfs/zfeature_common.c
         if name == 'Nexenta' and ver.startswith('4.'):
             features[('meta_devices', 'com.nexenta')].append((name, ver))
+    sources[name] = found
 
 os_sources = sources.copy()
 os_sources.pop(openzfs_key)