Avoid trying to load .netrc if $HOME isn't set.

This logged an error on Windows:

ERROR    misc       networkmanager:on_authentication_required:269 Unable to read the netrc file
Traceback (most recent call last):
  File "c:\python34\Lib\netrc.py", line 27, in __init__
    file = os.path.join(os.environ['HOME'], ".netrc")
  File "C:\Users\florian\buildbot\slave\win8\build\.tox\py34\lib\os.py", line 633, in __getitem__
    raise KeyError(key) from None
KeyError: 'HOME'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\florian\buildbot\slave\win8\build\qutebrowser\browser\network\networkmanager.py", line 262, in on_authentication_required
    net = netrc.netrc()
  File "c:\python34\Lib\netrc.py", line 29, in __init__
    raise OSError("Could not find .netrc: $HOME is not set")

Since this case is pretty common, we don't want to log it - and checking the
variable beforehand is easier than parsing the exception message.

This should fix the failing tests on Windows.

CHANGELOG.asciidoc

@@ -32,6 +32,8 @@ Fixed
 ~~~~~
 
 - Fixed completion for various config values when using `:set`.
+- Prevented an error being logged when a website with HTTP authentication was
+  opened on Windows.
 
 v0.5.0
 ------

qutebrowser/browser/network/networkmanager.py

@@ -19,6 +19,7 @@
 
 """Our own QNetworkAccessManager."""
 
+import os
 import collections
 import netrc
 
@@ -256,7 +257,10 @@ class NetworkManager(QNetworkAccessManager):
     def on_authentication_required(self, reply, authenticator):
         """Called when a website needs authentication."""
         user, password = None, None
-        if not hasattr(reply, "netrc_used"):
+        if not hasattr(reply, "netrc_used") and 'HOME' in os.environ:
+            # We'll get an OSError by netrc if 'HOME' isn't available in
+            # os.environ. We don't want to log that, so we prevent it
+            # altogether.
             reply.netrc_used = True
             try:
                 net = netrc.netrc()