A more reasonable error when importing js

[vadimkantorov]

I realize that only from js import something is supported, but a more reasonable error message for import js would be very helpful:

import js
Error: TypeError: object of type 'NoneType' has no len()

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/lib/python3.7/site-packages/pyodide.py", line 42, in eval_code
    exec(compile(mod, '<exec>', mode='exec'), ns, ns)
  File "<exec>", line 1, in <module>
SystemError: <JsImport object at 0x81c2e8> returned a result with an error set