Skip invalid files with a warning

[andreaskweber]

phpunit-merge-ouput:
     [exec] phpjunitmerge 1.0.1 by Andreas Weber
     [exec] 
     [exec] 
     [exec] 
     [exec]                                                                                                                                                                                                                          
     [exec]   [TheSeer\fDOM\fDOMException]                                                                                                                                                                                           
     [exec]   loading file '/var/lib/jenkins/jobs/ia-backend-md-trunk-nightly/workspace/build/phpunit/junit-api-affilinet.xml' failed.                                                                                               
     [exec]   [/var/lib/jenkins/jobs/ia-backend-md-trunk-nightly/workspace/build/phpunit/junit-api-affilinet.xml] [Line: 707 - Column: 147] Fatal Error 9: Input is not proper UTF-8, indicate encoding !Bytes: 0xE6 0xBB 0xFF 0xC3  
     [exec]   [/var/lib/jenkins/jobs/ia-backend-md-trunk-nightly/workspace/build/phpunit/junit-api-affilinet.xml] [Line: 89158 - Column: 109] Error 2: xmlSAX2Characters: huge text node                                             
     [exec]   [/var/lib/jenkins/jobs/ia-backend-md-trunk-nightly/workspace/build/phpunit/junit-api-affilinet.xml] [Line: 89158 - Column: 110] Fatal Error 5: Extra content at the end of the document                                
     [exec]                                                                                                                                                                                                                          
     [exec] 
     [exec] 
     [exec] phpjunitmerge [--names="..."] [--ignore="..."] dir file
     [exec] 
     [exec]