[generic] Set default-search to error

This prevents users from submitting bug reports where they mistyped a URL, and prevents me from getting a weird video when holding shift and thus searching for :Tds

youtube_dl/__init__.py

@@ -270,7 +270,7 @@ def parseOpts(overrideArguments=None):
     general.add_option(
         '--default-search',
         dest='default_search', metavar='PREFIX',
-        help='Use this prefix for unqualified URLs. For example "gvsearch2:" downloads two videos from google videos for  youtube-dl "large apple". By default (with value "auto") youtube-dl guesses.')
+        help='Use this prefix for unqualified URLs. For example "gvsearch2:" downloads two videos from google videos for  youtube-dl "large apple". Use the value "auto" to let youtube-dl guess. The default value "error" just throws an error.')
     general.add_option(
         '--ignore-config',
         action='store_true',

youtube_dl/extractor/generic.py

@@ -383,7 +383,7 @@ class GenericIE(InfoExtractor):
         if not parsed_url.scheme:
             default_search = self._downloader.params.get('default_search')
             if default_search is None:
-                default_search = 'auto_warning'
+                default_search = 'error'
 
             if default_search in ('auto', 'auto_warning'):
                 if '/' in url:
@@ -397,8 +397,13 @@ class GenericIE(InfoExtractor):
                                 expected=True)
                         else:
                             self._downloader.report_warning(
-                                'Falling back to youtube search for  %s . Set --default-search to "auto" to suppress this warning.' % url)
+                                'Falling back to youtube search for  %s . Set --default-search "auto" to suppress this warning.' % url)
                     return self.url_result('ytsearch:' + url)
+            elif default_search == 'error':
+                raise ExtractorError(
+                    ('%r is not a valid URL. '
+                     'Set --default-search "ytseach" (or run  youtube-dl "ytsearch:%s" ) to search YouTube'
+                    ) % (url, url), expected=True)
             else:
                 assert ':' in default_search
                 return self.url_result(default_search + url)