search: Print error message to stderr when not using TTY

[whoiswillma]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

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Address #8870 / start a conversation about how to address it.

• Surface searching pull requests with a --pull-request option.
• Raise an exception for when no formulae/casks are found to match the behavior of brew upgrade as mentioned in brew search doesn't return error as STDERR #8870 .

[MikeMcQuaid]

Looking good so far!

• Surface searching pull requests with a --pull-request option.

Is this mentioned in an open issue or just something you personally had a use for?

[whoiswillma]

Not mentioned in the issue: I just thought it was strange that the way you had to search for PRs was by entering the name of a formula that was recently deleted.

The less ideological but more realistic answer is that I wanted brew search to throw an exception to match the behavior of brew upgrade et al., but I couldn't throw an exception then print PR search results to replicate the current functionality.

[MikeMcQuaid]

Thanks again @whoiswillma!