board list now returns partial results in case of errors

[silvanocerza]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

Makes arduino-cli more resilient to errors.

• What is the current behavior?

Calling arduino-cli board list runs all discoveries installed, by default builtin:serial-discovery and builtin:mdns-discovery are installed and ran.

If one of those discoveries return an error the whole command fails.

• What is the new behavior?

arduino-cli board list now prints any error from discoveries and then eventual partial results obtains from other discoveries.

• Does this PR introduce a breaking change, and is
  titled accordingly?

None.

• Other information:

Fixes #1666.

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