Correct and simplify URLError exception handling

Julien Palard pointed out that the way URLError exceptions were
being manually cobbled into the stderr stream wasn't quite working
(thanks!), but it was also unnecessarily complicated for reasons I
don't recall now. Rip most of it out and just go with a basic
catch/error/re-raise there instead.

weather.py

@@ -221,18 +221,10 @@ def get_uri(
             data = urlopen(uri).read().decode("utf-8")
         except URLError:
             if ignore_fail: return ""
-            else:
+            import os, sys
-                import os, sys, traceback
+            sys.stderr.write("%s error: failed to retrieve\n   %s\n\n" % (
-                message = "%s error: failed to retrieve\n   %s\n   %s" % (
+                os.path.basename( sys.argv[0] ), uri))
-                        os.path.basename( sys.argv[0] ),
+            raise
-                        uri,
-                        traceback.format_exception_only(
-                            sys.exc_type,
-                            sys.exc_value
-                        )[0]
-                    )
-                sys.stderr.write(message)
-                sys.exit(1)
         # Some data sources are HTML with the plain text wrapped in pre tags
         if "<pre>" in data:
             data = data[data.find("<pre>")+5:data.find("</pre>")]