improve error message

lib/ecdsa.js

@@ -114,7 +114,7 @@ ECDSA.prototype.sig2pubkey = function() {
 
 ECDSA.prototype.sigError = function() {
   if (!Buffer.isBuffer(this.hashbuf) || this.hashbuf.length !== 32)
-    return 'Invalid hash';
+    return 'hashbuf must be a 32 byte buffer';
 
   try {
     this.keypair.pubkey.validate();

test/ecdsa.js

@@ -105,7 +105,7 @@ describe("ECDSA", function() {
 
     it('should return an error if the hash is invalid', function() {
       var ecdsa = new ECDSA();
-      ecdsa.sigError().should.equal('Invalid hash');
+      ecdsa.sigError().should.equal('hashbuf must be a 32 byte buffer');
     });
 
     it('should return an error if the pubkey is invalid', function() {