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/plugins/browser/main.py

@@ -6,7 +6,15 @@ import os
 import string
 
 if os.name == 'nt':
-    import win32file
+    import imp
+    try:
+        imp.find_module('win32file')
+    except:
+        # todo:: subclass ImportError for missing dependencies, vs. broken plugins?
+        raise ImportError("Missing the win32file module, which is a part of the prerequisite \
+            pywin32 package. You can get it from http://sourceforge.net/projects/pywin32/files/pywin32/");
+    else:
+        import win32file
 
 class FileBrowser(Plugin):