test: Print hugr-cli's errors on python tests

[aborgna-q]

Prints the error returned by hugr-cli when something fails, so we get better feedback.

Before:

subprocess.CalledProcessError: Command '['/Users/agustinborgna/src/hugr/target/debug/hugr', '-']' returned non-zero exit status 1.

After:

RuntimeError: Error validating HUGR: Parent node Node(0) has extensions [] that are too restrictive for child node Node(5), they must include child extensions [IdentList("quantum.tket2")]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.24%. Comparing base (cb84b84) to head (0d77777).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1310   +/-   ##
=======================================
  Coverage   87.24%   87.24%           
=======================================
  Files         107      107           
  Lines       19586    19586           
  Branches    17322    17322           
=======================================
  Hits        17088    17088           
  Misses       1714     1714           
  Partials      784      784           

Flag	Coverage Δ
python	91.43% <ø> (ø)

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