Better error message

If the blob looks like it was base64 encoded,
note this in the error message.

Author: seebees

modules/serialize/src/deserialize_factory.ts

@@ -77,7 +77,8 @@ export function deserializeFactory<Suite extends AlgorithmSuite> (
     const version = dataView.getUint8(0)
     const type = dataView.getUint8(1)
     /* Precondition: version and type must be the required values. */
-    needs(version === 1 && type === 128, 'Malformed Header')
+    needs(version === 1 && type === 128,
+      version === 65 && type === 89 ? 'Malformed Header: This blob may be base64 encoded.' : 'Malformed Header.' )
 
     const suiteId = <AlgorithmSuiteIdentifier>dataView.getUint16(2, false) // big endian
     /* Precondition: suiteId must match supported algorithm suite */