Improve error message for undefined inputs

[orasimus]

If I have code referencing an input like:

import valohai

valohai.prepare(
    step='steppi',
    default_inputs={
        'inputti': 's3://bucket/file',
    },
)

print(valohai.inputs('inputti').path())

But then I forget to generate or manually update the valohai.yaml to include the input inputti, the error I get is:

15:28:06  raise InvalidSchema(f"No connection adapters were found for {url!r}")
15:28:06 requests.exceptions.InvalidSchema: No connection adapters were found for 's3://bucket/file'

This is not helpful in understanding that I'm trying to access an input which hasn't been defined.