Migrate exception message should give the index of the process plugin in the pipeline

Quick fix, not sure about the message format!

Seemed to have caused some test failures.

Have not reviewed.

Akhil Babu → made their first commit to this issue’s fork.

Tests were failing due to mismatch in message after the change. Updated them.

Few more tests should be updated

There is a strange Functionaljavscript test failure as well.

   1) Drupal\FunctionalJavascriptTests\Ajax\AjaxTest::testAjaxFocus
    Failed asserting that two strings are equal.
    --- Expected
    +++ Actual
    @@ @@
    -'edit-textfield-2'
    +'edit-textfield-3'

Pipeline fixed after rebase. Moving to needs review

For good practice we should have a complete issue summary.