Dependency verification report should be more explicit when key download failed

[eskatos]

This issue is about enhancing the dependency verification report to better explain failures to download a verification key.

Expected Behavior

When dependency verification fails because a key couldn't be downloaded from a server, the report should say something like:

Key xyz (not found or failed to download) couldn't be downloaded from any key server

Current Behavior

When dependency verification fails because a key couldn't be downloaded from a server, the report currently says:

Key xyz (not found) couldn't be found in any key server

even though they key is present in the key server but the underlying problem is that the key server didn't respond. This is confusing.

Context

Happened on CI

• https://builds.gradle.org/viewLog.html?buildId=34165816
• https://e.grdev.net/s/mpz6ivwpghjm2/failure?openFailures=WzBd&openStackTraces=WzFd#top=0
• https://builds.gradle.org/repository/download/Gradle_Check_Quick_1_bucket38/34165816:id/dependency-verification-reports/at-1587974709376/dependency-verification-report.html

Steps to Reproduce

Not that easy with a valid key identifier because key servers sporadically fail to respond.
Using a non existent key server URL should do.