Consistent exception handling

What does this MR do?

Handle errors thrown by DAST using a consistent strategy:

• Rename DASTExceptionHandler to DASTErrorHandler
• The only class to call logger.error should be the DASTErrorHandler
• The DASTErrorHandler defines exception types that should be logged without a stacktrace
• The ScanScriptWrapper should not call system.exit
• Error types are fairly general, the message in the constructor is very specific

What are the relevant issue numbers?

gitlab-org/gitlab#214240 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer