JENKINS-6415 fix and fix that fail build when some files were left in directory on workspace but there was no .svn directory and we wanted to update it

Example of problem when some files were in workspace:

svn co http://repository/dir
.svn/
other_file
/somefolder/
-  .svn/
- a_file

after clean(deleting .svn directories) in workspace

.svn/
other_file
/somefolder/
- a_file

There was error but subversion plugin went over that and pulled other externals if any and build without failure. 

With those changes now build should failure when error occurs during update/checkout of svn:externals. But the major fix is in JENKINS-6415 which silently omited problems with externals.