replaygain: More descriptive backend errors

This avoids a bare `except:` to catch specific errors and give better messages
when they fail. Should provide more insight to this trouble:
https://groups.google.com/d/msg/beets-users/6d-TOg0SuPY/iL39tmo3rqUJ

Also related to #874.

Feedback from @yevgenybezman or others more familiar with the backend would be
welcome.

beetsplug/replaygain.py

@@ -273,7 +273,17 @@ class GStreamerBackend(object):
 
         try:
             import gi
+        except ImportError:
+            raise FatalReplayGainError(
+                "Failed to load GStreamer: python-gi not found"
+            )
+
+        try:
             gi.require_version('Gst', '1.0')
+        except ValueError as e:
+            raise FatalReplayGainError(
+                "Failed to load GStreamer 1.0: {0}".format(e)
+            )
 
         from gi.repository import GObject, Gst, GLib
         # Calling GObject.threads_init() is not needed for
@@ -282,10 +292,6 @@ class GStreamerBackend(object):
             warnings.simplefilter("ignore")
             GObject.threads_init()
         Gst.init([sys.argv[0]])
-        except:
-            raise FatalReplayGainError(
-                "Failed to load GStreamer; check that python-gi is installed"
-            )
 
         self.GObject = GObject
         self.GLib = GLib
@@ -498,7 +504,7 @@ class ReplayGainPlugin(BeetsPlugin):
             )
         except (ReplayGainError, FatalReplayGainError) as e:
             raise ui.UserError(
-                'An error occurred in backend initialization: {0}'.format(e)
+                'replaygain initialization failed: {0}'.format(e)
             )
 
     def track_requires_gain(self, item):