build(python): fail nox sessions if a python version is missing (#982)

Nox's default behavior is to quietly skip if a python interpreter is missing. https://nox.thea.codes/en/stable/usage.html#failing-sessions-when-the-interpreter-is-missing

Author: busunkim96

synthtool/gcp/templates/python_library/noxfile.py.j2

@@ -41,6 +41,10 @@ nox.options.sessions = [
     "docs",
 ]
 
+# Error if a python version is missing
+nox.options.error_on_missing_interpreters = True
+
+
 @nox.session(python=DEFAULT_PYTHON_VERSION)
 def lint(session):
     """Run linters.