Add helpful instructions to direct user if cmdstanr is not installed #350

All 89 tests pass, 31 skipped in 5m 20s

1 files ±0 40 suites ±0 5m 20s ⏱️ -6s
120 tests ±0 89 ✅ ±0 31 💤 ±0 0 ❌ ±0
849 runs ±0 818 ✅ ±0 31 💤 ±0 0 ❌ ±0

Results for commit 89b508a. ± Comparison against earlier commit 52b8caf.