Fix #71 Do signature change error msg

Update the error handling for Call.Do in the case where
the argument passed to Call.Do does not match expectations.

* panic if the argument is not a function
* panic if the number of input arguments do not match those expected by Call
* panic if the types of the input arguments do not match those expected
by Call
* panic if the number of return arguments do not match those expected by
Call
* panic if the types of return arguments do not match those expected by
Call

gomock/call.go

@@ -19,6 +19,8 @@ import (
 	"reflect"
 	"strconv"
 	"strings"
+
+	"github.com/golang/mock/gomock/internal/calldo"
 )
 
 // Call represents an expected call to a mock.
@@ -135,9 +137,20 @@ func (c *Call) DoAndReturn(f interface{}) *Call {
 // return values call DoAndReturn.
 // It takes an interface{} argument to support n-arity functions.
 func (c *Call) Do(f interface{}) *Call {
-	// TODO: Check arity and types here, rather than dying badly elsewhere.
 	v := reflect.ValueOf(f)
 
+	switch v.Kind() {
+	case reflect.Func:
+		mt := c.methodType
+
+		ft := v.Type()
+		if err := calldo.ValidateInputAndOutputSig(ft, mt); err != nil {
+			panic(err)
+		}
+	default:
+		panic("Do: argument must be a function")
+	}
+
 	c.addAction(func(args []interface{}) []interface{} {
 		vargs := make([]reflect.Value, len(args))
 		ft := v.Type()