[dotnet] Indicate end of output taken from selenium manager

[nvborisenko]

Description

When selenium manager fails to do something, we raise output in exception. This PR improves slightly output to indicate where output starts and ends.

Motivation and Context

Improve readability of SM process output.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.