Better error display in package-client

Addressing: https://github.com/meteor/meteor/issues/3977

HttpHelpers.getUrl sometimes throws an error, and sometimes throws a string. The right
thing to do is to fix getUrl everywhere, but for now, let's get better error handling
in package-client.js

tools/package-client.js

@@ -328,7 +328,8 @@ var uploadFile = function (putUrl, filepath) {
       bodyStreamLength: size
     });
   } catch (err) {
-    buildmessage.error(err.error.toString());
+    // XXX: getUrl's error handling is terrible and we should fix it there.
+    buildmessage.error(typeof err === "string" ? err : err.error.toString());
     return false;
   } finally {
     rs.close();