Summary provides more guidance to user when no API data is available

[erichaagdev]

Currently, if the user has no API access or the calls fail, the user may see some errors and the Performance Characteristics section lists <unknown> for all values in orange text.

This can be quite intimidating and often leads the user to believe the entire experiment failed.

We should make this output more friendly and provide guidance to the user on how to address the issue. For example:

Performance Characteristics
---------------------------
The 'Performance Characteristics' section was not populated because ..... See http://gradle.com/bvs/main/Gradle.md#authenticating-with-develocity

We could also opt to hide the Performance Characteristics section completely and have it as a warning. For example:

...
Build scan first build:      https://develocity.example.com/s/abcdefg
Build scan first build:      https://develocity.example.com/s/hijklmnop

WARNING: The 'Performance Characteristics' section was not populated because ..... See http://gradle.com/bvs/main/Gradle.md#authenticating-with-develocity

Investigation Quick Links
-------------------------
Task execution overview:      https://develocity.example.com/s/hijklmnop/performance/execution
...