Better error message for missing inputs.

[copybara-service]

Better error message for missing inputs.

For the added test, this will now result in the following error:

tf2jax._src.tf2jax.MissingInputError:
Some input(s) are missing (entries that map to dtype[shape] annotations in the following structure):
{
"x": ".",
"params": {
"a": ".",
"b": "float32[2]"
}
}