arduino-cli reports errors as plain text or, with --format json, as a JSON
object on stderr. We forwarded that blob verbatim, so on a fresh install
with no library index yet (and no way through the firewall/proxy to
downloads.arduino.cc) the in-app agent told the user "the library server
returns a JSON error" ten times in a row.
humanize_cli_error() maps the known shapes — index download failed,
library zip download failed, library not found — to a single sentence
that says what to check, and keeps the raw text under raw_error. Applied
to search_libraries (rc != 0 and non-JSON stdout) and install_library.
Tests use the real arduino-cli 1.0.4 output captured offline.
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