0.2.9 fails: ModuleNotFoundError: No module named 'jaxlib'
```
Python 3.7.9 (default, Dec 12 2020, 01:25:17) 
[Clang 8.0.1 (tags/RELEASE_801/final 366581)] on freebsd12
Type "help", "copyright", "credits" or "license" for more information.
```

These lines:
```
import jax.numpy as jnp
from jax import grad, jit, vmap
```
fail:
```
$ python3.7 j.py 
Traceback (most recent call last):
  File "j.py", line 1, in <module>
    import jax.numpy as jnp
  File "/usr/local/lib/python3.7/site-packages/jax/__init__.py", line 22, in <module>
    from .api import (
  File "/usr/local/lib/python3.7/site-packages/jax/api.py", line 39, in <module>
    from . import core
  File "/usr/local/lib/python3.7/site-packages/jax/core.py", line 31, in <module>
    from . import dtypes
  File "/usr/local/lib/python3.7/site-packages/jax/dtypes.py", line 32, in <module>
    from .lib import xla_client
  File "/usr/local/lib/python3.7/site-packages/jax/lib/__init__.py", line 23, in <module>
    import jaxlib
ModuleNotFoundError: No module named 'jaxlib'
```

installed from the FreeBSD port
OS: FreeBSD 12.2

Downstream bug report: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252871
diff --git a/jax/lib/__init__.py b/jax/lib/__init__.py
index e3a2cd8f1..894e6a785 100644
--- a/jax/lib/__init__.py
+++ b/jax/lib/__init__.py
@@ -20,7 +20,13 @@ __all__ = [
   'pocketfft', 'pytree', 'tpu_client', 'version', 'xla_client'
 ]
 
-import jaxlib
+try:
+  import jaxlib
+except ModuleNotFoundError as err:
+  raise ModuleNotFoundError(
+    'jax requires jaxlib to be installed. See '
+    'https://github.com/google/jax#installation for installation instructions.'
+    ) from err
 
 # Must be kept in sync with the jaxlib version in build/test-requirements.txt
 _minimum_jaxlib_version = (0, 1, 60)
['jax/lib/__init__.py']