Pass plugin errors back out to the user.

Closes: #971

letsencrypt/cli.py

@@ -351,7 +351,8 @@ def diagnose_configurator_problem(cfg_type, requested, plugins):
             msg = "The requested {0} plugin does not appear to be installed".format(requested)
         else:
             msg = ("The {0} plugin is not working; there may be problems with "
-                   "your existing configuration").format(requested)
+                   "your existing configuration.\nThe error was: {1}"
+                   .format(requested, plugins[requested].problem))
     elif cfg_type == "installer":
         if os.path.exists("/etc/debian_version"):
             # Debian... installers are at least possible

letsencrypt/plugins/disco.py

@@ -120,6 +120,12 @@ class PluginEntryPoint(object):
         """Is plugin misconfigured?"""
         return isinstance(self._prepared, errors.MisconfigurationError)
 
+    @property
+    def problem(self):
+        """Return the Exception raised during plugin setup, or None if all is well"""
+        if isinstance(self._prepared, Exception):
+            return self._prepared
+
     @property
     def available(self):
         """Is plugin available, i.e. prepared or misconfigured?"""