Improve error message when target url is not provided

Problem

When performing DAST API scans using GraphQL, if the user omits the DAST_API_TARGET_URL variable the resulting error message is vague and not actionable.

• Provide DAST_API_GRAPHQL, but not DAST_API_TARGET_URL
• Error message: 01:42:41 [ERR] DAST API: . Error, no base URL to target was found.

Proposal

Improve the error message to be clearer, and provide guidance on what the user needs todo.