[GTK] Fix showing correct error on libtorrent import error

The exception string "No module named libtorrent" was changed to
"No module named 'libtorrent'" in python 3.3, which results in a
"unknown Import Error" message being displayed instead of the
message meant for libtorrent import error.

Change to raising LibtorrentImportError in _libtorrent.py and
catch this error to display libtorrent specific import errors.

deluge/_libtorrent.py

@@ -18,16 +18,21 @@ Example:
 from __future__ import unicode_literals
 
 from deluge.common import VersionSplit, get_version
+from deluge.error import LibtorrentImportError
 
 try:
     import deluge.libtorrent as lt
 except ImportError:
-    import libtorrent as lt
+    try:
+        import libtorrent as lt
+    except ImportError as ex:
+        raise LibtorrentImportError('No libtorrent library found: %s' % (ex))
+
 
 REQUIRED_VERSION = '1.1.2.0'
 LT_VERSION = lt.__version__
 
 if VersionSplit(LT_VERSION) < VersionSplit(REQUIRED_VERSION):
-    raise ImportError(
+    raise LibtorrentImportError(
         'Deluge %s requires libtorrent >= %s' % (get_version(), REQUIRED_VERSION)
     )

deluge/error.py

@@ -94,3 +94,7 @@ class AuthenticationRequired(_UsernameBasedPasstroughError):
 
 class AuthManagerError(_UsernameBasedPasstroughError):
     pass
+
+
+class LibtorrentImportError(ImportError):
+    pass

deluge/ui/gtk3/gtkui.py

@@ -45,7 +45,7 @@ from deluge.common import (
     windows_check,
 )
 from deluge.configmanager import ConfigManager, get_config_dir
-from deluge.error import DaemonRunningError
+from deluge.error import DaemonRunningError, LibtorrentImportError
 from deluge.i18n import I18N_DOMAIN, set_language, setup_translation
 from deluge.ui.client import client
 from deluge.ui.hostlist import LOCALHOST
@@ -313,8 +313,8 @@ class GtkUI(object):
                 'A Deluge daemon (deluged) is already running.\n'
                 'To use Standalone mode, stop local daemon and restart Deluge.'
             )
-        except ImportError as ex:
+        except LibtorrentImportError as ex:
-            if 'No module named libtorrent' in str(ex):
+            if 'libtorrent library not found' in str(ex):
                 err_msg = _(
                     'Only Thin Client mode is available because libtorrent is not installed.\n'
                     'To use Standalone mode, please install libtorrent package.'
@@ -322,9 +322,17 @@ class GtkUI(object):
             else:
                 log.exception(ex)
                 err_msg = _(
-                    'Only Thin Client mode is available due to unknown Import Error.\n'
+                    'Only Thin Client mode is available due to libtorrent import error: %s\n'
                     'To use Standalone mode, please see logs for error details.'
+                    % (str(ex))
                 )
+
+        except ImportError as ex:
+            log.exception(ex)
+            err_msg = _(
+                'Only Thin Client mode is available due to unknown Import Error.\n'
+                'To use Standalone mode, please see logs for error details.'
+            )
         except Exception as ex:
             log.exception(ex)
             err_msg = _(