Always show device discovery diagnostics in "flutter devices"

[jmagman]

Description

Always show diagnostics in flutter devices. Currently only shown if no devices are found (not great when you have desktop and web features on and devices are permanently available).

flutter$ flutter devices
1 connected device:

Pixel 4 • 98281FFAZ005T6 • android-arm64 • Android 10 (API 29)

vs

$ flutter devices
1 connected device:

Pixel 4 • 98281FFAZ005T6 • android-arm64 • Android 10 (API 29)

• Error: iPhone is not paired with your computer. To use iPhone with Xcode, unlock it and choose to trust this computer when prompted. (code -9)

Related Issues

Fixes #54388

Tests

devices_test

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.

[jmagman]

Tip: this one is easier to review with whitespace changes ignored: