chore(toxgen): Check availability of pip and add detail to exceptions

[alexander-alderman-webb]

Description

Exit populate_tox.py early if pip module is not available, and add a suggestion for running the script.

Issues

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
27850	1	27849	2208

View the full list of 1 ❄️ flaky test(s)

tests.profiler.test_continuous_profiler::test_continuous_profiler_auto_start_and_manual_stop[non-experiment-start_profile_session/stop_profile_session (deprecated)-thread]

Flake rate in main: 100.00% (Passed 0 times, Failed 31 times)

Stack Traces | 0.168s run time

tests/profiler/test_continuous_profiler.py:270: in test_continuous_profiler_auto_start_and_manual_stop
    assert_single_transaction_with_profile_chunks(envelopes, thread)
tests/profiler/test_continuous_profiler.py:147: in assert_single_transaction_with_profile_chunks
    assert len(items["profile_chunk"]) > 0
E   assert 0 > 0
E    +  where 0 = len([])

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