[dotnet] Treat unsuccessful http responses as errored commands

[nvborisenko]

Description

Currently http responses are treated as unsuccessful only based on body content. This PR verifies http status code first, avoiding a process of parsing body content.

Motivation and Context

Related to #11713, according https://www.w3.org/TR/webdriver2/#errors

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.