ui_test_core: make verifyImageMatchSnapshot continue execution

Introduce another assertImageMatchSnapshot which stops on failure,
following the behavior of other selenium assertions.

Also output more details when the dimensions are different.
18 jobs for feat/selenium-unexpected-success in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

05:23:23

passed ERP5.UnitTest-Master

06:41:57

failed ERP5.UnitTest-Zope2

07:45:26

passed SlapOS.Eggs.UnitTest-Master.Python2

02:21:03

passed SlapOS.Eggs.UnitTest-Master.Python3

02:01:35

passed Wendelin.UnitTest-Master

04:34:54

passed ERP5.CodingStyleTest-Master

00:48:58

passed ERP5.CodingStyleTest-Master

00:44:06

failed ERP5.UnitTest-Master

01:56:59

failed ERP5.UnitTest-Master

01:12:04

failed ERP5.UnitTest-Zope2

02:15:50

failed ERP5.UnitTest-Zope2

01:37:45

passed SlapOS.Eggs.UnitTest-Master.Python2

00:15:29

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:52

passed SlapOS.Eggs.UnitTest-Master.Python3

00:16:25

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:48

passed Wendelin.UnitTest-Master

00:32:18

passed Wendelin.UnitTest-Master

00:26:12