Simplify library existence detection.

src/zc/buildout/networkcache.py

@@ -20,6 +20,13 @@ import re
 import urllib
 import urlparse
 
+try:
+    from slapos.libnetworkcache import NetworkcacheClient, UploadError, \
+      DirectoryNotFound
+except ImportError:
+    LIBNETWORKCACHE_ENABLED = False
+else:
+    LIBNETWORKCACHE_ENABLED = True
 
 _md5_re = re.compile(r'md5=([a-f0-9]+)')
 
@@ -52,12 +59,8 @@ def download_network_cached(sha_dir, sha_cache, path, url, logger, md5sum=None):
 
     return True if download succeeded.
     """
-    try:
-      from slapos.libnetworkcache import NetworkcacheClient, UploadError, \
-                                   DirectoryNotFound
-    except:
-      return False
-
+    if not LIBNETWORKCACHE_ENABLED:
+        return False
 
     if sha_cache in (None, '',):
         # Not able to use network cache
@@ -94,11 +97,8 @@ def download_network_cached(sha_dir, sha_cache, path, url, logger, md5sum=None):
 
 def upload_network_cached(sha_cache, sha_dir, external_url, path, logger):
     """Upload file to a network cache server"""
-    try:
-      from slapos.libnetworkcache import NetworkcacheClient, UploadError, \
-                                   DirectoryNotFound
-    except:
-      return False
+    if not LIBNETWORKCACHE_ENABLED:
+        return False
 
     if sha_cache in [None, ''] or sha_dir in [None, '']:
         logger.debug(