adjusted regex for matching external stack errors when SLS_DEBUG is set

[MHekert]

What did you implement:

When running sls deploy with SLS_DEBUG=* errors messages have additional text and are not correctly detected by current regex. e.g

when run sls deploy without SLS_DEBUG=* error message is following:

Stack with id STACK_NAME does not exist

and when SLS_DEBUG=* is set it has following format:

ValidationError: Stack with id STACK_NAME does not exist\nSTACK_TRACE

How did you implement it:

I removed requirement for matching a beginning or ending of string and added some extra text to prevent false positive matching.

How can we verify it:

Set externalStack to true

custom:
  alerts:
    externalStack: true
    stages:
      - prod
      - test
      - dev

Run sls deploy with and without SLS_DEBUG=* set for stage present in the list and for stage not present in the list.