[Improvement] Add placeholder for failed test without message field

[svenvg93]

📃 Description

Sometimes, the Ookla CLI fails to execute the test due to an unknown error. Unfortunately, in such cases, the CLI does not return a complete JSON error response. Instead, it only provides:

{
    "error": "Cannot read: "
}

Our current logic expects a complete JSON response containing a message field. This PR introduces a placeholder error message for these cases, ensuring that failed tests are not left without an error message.

As we cannot do anything about the reason of the failure this;

• closes Failed run has no log or information #1928
• closes Resource Temporarily Unavailable  #1862

🪵 Changelog

➕ Added

• A placeholder message for failed cases without a message field.