feat: export AssertionError and ValidationError

Add ValidationError to help distinguish between unrecoverable errors
while decoding an `.sb` file and encoding to Scratch 2 data.

Export AssertionError and ValidationError so users of the converter can
know when an error is from validating the Scratch 1 file (as in it
likely is not an Scratch 1 file) or from making sure the Scratch 1 file
data matches expected ranges (like not making memory buffers that are
too big).

src/index.js

@@ -1 +1,2 @@
 export {SB1File} from './sb1-file';
+export {AssertionError, ValidationError} from './util/assert';

src/sb1-file-packets.js

@@ -27,7 +27,7 @@ class SB1Signature extends Packet.extend({
      * @throws {AssertionError} Throws if it is not valid.
      */
     validate () {
-        assert(
+        assert.validate(
             this.equals({version: 'ScratchV01'}) ||
             this.equals({version: 'ScratchV02'}),
             'Invalid Scratch file signature.'
@@ -45,7 +45,7 @@ class SB1Header extends Packet.extend({
     numObjects: Uint32BE
 }) {
     validate () {
-        assert(
+        assert.validate(
             this.equals({
                 ObjS: 'ObjS',
                 ObjSValue: 1,

src/util/assert.js

@@ -3,11 +3,21 @@
  */
 class AssertionError extends Error {}
 
+/**
+ * A `scratch-sb1-converter` validation error.
+ */
+class ValidationError extends AssertionError {}
+
 const assert = function (test, message) {
     if (!test) throw new AssertionError(message);
 };
 
-export {
+assert.validate = function (test, message) {
-    AssertionError,
+    if (!test) throw new ValidationError(message);
-    assert
+};
+
+export {
+    assert,
+    AssertionError,
+    ValidationError
 };