Improve transaction assertion messages

lib/transaction/transaction.js

@@ -393,13 +393,13 @@ Transaction.prototype.fromObject = function fromObject(arg) {
 
 Transaction.prototype._checkConsistency = function(arg) {
   if (!_.isUndefined(this._changeIndex)) {
-    $.checkState(this._changeScript);
-    $.checkState(this.outputs[this._changeIndex]);
+    $.checkState(this._changeScript, 'Change script is expected.');
+    $.checkState(this.outputs[this._changeIndex], 'Change index points to undefined output.');
     $.checkState(this.outputs[this._changeIndex].script.toString() ===
-      this._changeScript.toString());
+      this._changeScript.toString(), 'Change output has an unexpected script.');
   }
   if (arg && arg.hash) {
-    $.checkState(arg.hash === this.hash, 'Hash in object does not match transaction hash');
+    $.checkState(arg.hash === this.hash, 'Hash in object does not match transaction hash.');
   }
 };
 
@@ -1050,7 +1050,7 @@ Transaction.prototype.removeInput = function(txId, outputIndex) {
  * @return {Transaction} this, for chaining
  */
 Transaction.prototype.sign = function(privateKey, sigtype) {
-  $.checkState(this.hasAllUtxoInfo());
+  $.checkState(this.hasAllUtxoInfo(), 'Not all utxo information is available to sign the transaction.');
   var self = this;
   if (_.isArray(privateKey)) {
     _.each(privateKey, function(privateKey) {