sslutil: display a better error message when CA file loading fails

Before, sslcontext.load_verify_locations() would raise a
ssl.SSLError which would be caught further up the stack and converted
to a urlerror. By that time, we lost track of what actually errored.

Trapping the error here gives users a slightly more actionable error
message.

The behavior between Python <2.7.9 and Python 2.7.9+ differs. This
is because our fake SSLContext class installed on <2.7.9 doesn't
actually do anything during load_verify_locations: it defers actions
until wrap_socket() time. Unfortunately, a number of errors can occur
at wrap_socket() time and we're unable to ascertain what the root
cause is. But that shouldn't stop us from providing better error
messages to people running a modern and secure Python version.

mercurial/sslutil.py

@@ -268,7 +268,12 b' def wrapsocket(sock, keyfile, certfile, '
         sslcontext.load_cert_chain(certfile, keyfile, password)
 
     if settings['cafile'] is not None:
-        sslcontext.load_verify_locations(cafile=settings['cafile'])
+        try:
+            sslcontext.load_verify_locations(cafile=settings['cafile'])
+        except ssl.SSLError as e:
+            raise error.Abort(_('error loading CA file %s: %s') % (
+                              settings['cafile'], e.args[1]),
+                              hint=_('file is empty or malformed?'))
         caloaded = True
     elif settings['allowloaddefaultcerts']:
         # This is a no-op on old Python.

tests/test-https.t

@@ -62,9 +62,16 b' Specifying a per-host certificate file t'
 A malformed per-host certificate file will raise an error
 
   $ echo baddata > badca.pem
+#if sslcontext
+  $ hg --config hostsecurity.localhost:verifycertsfile=badca.pem clone https://localhost:$HGPORT/
+  abort: error loading CA file badca.pem: * (glob)
+  (file is empty or malformed?)
+  [255]
+#else
   $ hg --config hostsecurity.localhost:verifycertsfile=badca.pem clone https://localhost:$HGPORT/
   abort: error: * (glob)
   [255]
+#endif
 
 A per-host certificate mismatching the server will fail verification
 
@@ -183,10 +190,19 b' variables in the filename'
 empty cacert file
 
   $ touch emptycafile
+
+#if sslcontext
+  $ hg --config web.cacerts=emptycafile -R copy-pull pull
+  pulling from https://localhost:$HGPORT/
+  abort: error loading CA file emptycafile: * (glob)
+  (file is empty or malformed?)
+  [255]
+#else
   $ hg --config web.cacerts=emptycafile -R copy-pull pull
   pulling from https://localhost:$HGPORT/
   abort: error: * (glob)
   [255]
+#endif
 
 cacert mismatch