Improve error message when reading garbage data

In TurboWarp, these messages can be seen by end-users.
The guidance to go to stuk.github.io/jszip is not helpful for users.
Let's replace it with something more meaningful for us and them.

Author: GarboMuffin

lib/zipEntries.js

@@ -164,8 +164,7 @@ ZipEntries.prototype = {
             var isGarbage = !this.isSignature(0, sig.LOCAL_FILE_HEADER);
 
             if (isGarbage) {
-                throw new Error("Can't find end of central directory : is this a zip file ? " +
-                                "If it is, see https://stuk.github.io/jszip/documentation/howto/read_zip.html");
+                throw new Error("Corrupted zip: can't find zip signature or end of central directory");
             } else {
                 throw new Error("Corrupted zip: can't find end of central directory");
             }

test/asserts/load.js

@@ -601,7 +601,8 @@ QUnit.module("load", function () {
                 assert.ok(false, "no exception were thrown");
                 done();
             }, function failure(e) {
-                assert.ok(e.message.match("stuk.github.io/jszip/documentation"), "the error message is useful");
+                assert.ok(e.message.match("can't find zip signature or end of central directory"), "the error message is useful");
+                assert.notOk(e.message.match("stuk.github.io/jszip/documentation"), "the error message does not reference jszip website");
                 done();
             });
     });