fix callserrors with empty msg

[codyoss]

Check exhaustedNum and callsErrors.len() , if it's true, fill with error msg

There is a case where matches will return nil for an ordering error.
e.g.

gomock.InOrder(
  mockFoo.EXPECT().Foo().AnyTimes(),
  mockBar.EXPECT().Bar().AnyTimes(),
  mockBaz.EXPECT().Baz().AnyTimes(),
)

mockFoo.Foo()
mockBar.Bar()
mockFoo.Foo()

This will produce an error message with no additional information following because: (as reported in the issue)

err will always be nil

mock/gomock/callset.go

Line 85 in 1bf84fa

if err := call.matches(args); err != nil {

len(exhausted) will be 1 and len(expected) will be 0

mock/gomock/callset.go

Line 90 in 1bf84fa

if len(expected)+len(exhausted) == 0 {

Originally posted by @cvgw in #292 (comment)