add wrapper around the `EqualError` function

* add a `*Assertions` wrapper around the `EqualError` function
* add tests

assert/forward_assertions.go

@@ -191,3 +191,16 @@ func (a *Assertions) NoError(theError error, msgAndArgs ...interface{}) bool {
 func (a *Assertions) Error(theError error, msgAndArgs ...interface{}) bool {
 	return Error(a.t, theError, msgAndArgs...)
 }
+
+// EqualError asserts that a function returned an error (i.e. not `nil`)
+// and that it is equal to the provided error.
+//
+//   actualObj, err := SomeFunction()
+//   if assert.Error(err, "An error was expected") {
+//	   assert.Equal(err, expectedError)
+//   }
+//
+// Returns whether the assertion was successful (true) or not (false).
+func (a *Assertions) EqualError(theError error, errString string, msgAndArgs ...interface{}) bool {
+	return EqualError(a.t, theError, errString, msgAndArgs...)
+}

assert/forward_assertions_test.go

@@ -267,6 +267,23 @@ func TestErrorWrapper(t *testing.T) {
 
 }
 
+func TestEqualErrorWrapper(t *testing.T) {
+	assert := New(t)
+	mockAssert := New(new(testing.T))
+
+	// start with a nil error
+	var err error
+	assert.False(mockAssert.EqualError(err, ""),
+		"EqualError should return false for nil arg")
+
+	// now set an error
+	err = errors.New("some error")
+	assert.False(mockAssert.EqualError(err, "Not some error"),
+		"EqualError should return false for different error string")
+	assert.True(mockAssert.EqualError(err, "some error"),
+		"EqualError should return true")
+}
+
 func TestEmptyWrapper(t *testing.T) {
 	assert := New(t)
 	mockAssert := New(new(testing.T))