Throw a useful error in the log when FileBrowser fails to load because of missing pywin32

This is a quick hack so that anyone else who runs this from source doesn't have to spend
the time I spent tracking down why directory.list failed silently.

There are two options that are much cleaner that come to mind:

- Subclass ImportException so as to differentiate missing requirements from
  parse errors etc.
- Provide a method for plugins to list their requirements, so that
  the loader can be the one to use imp.find_module().

Using imp.find_module() seems wise, either way.

couchpotato/core/loader.py

@@ -51,8 +51,13 @@ class Loader(object):
                     did_save += self.loadSettings(m, module_name, save = False)
 
                     self.loadPlugins(m, plugin.get('name'))
-                except ImportError:
-                    log.debug('Import error, remove the empty folder: %s' % plugin.get('module'))
+                except ImportError as e:
+                    # todo:: subclass ImportError for missing requirements.
+                    if (e.message.lower().startswith("missing")):
+                        log.error(e.message)
+                        pass
+                    # todo:: this needs to be more descriptive.
+                    log.error('Import error, remove the empty folder: %s' % plugin.get('module'))
                 except:
                     log.error('Can\'t import %s: %s' % (module_name, traceback.format_exc()))