input: improve error messages when retrieving slot values

Having one error message for all three failure types makes it harder to
work out what's going wrong when looking at logs, so be more specific.
While we're at it, specify the human-readable name of the axis we
couldn't retrieve a value for.

Test: $ m
Bug: 438570964
Flag: EXEMPT refactor
Change-Id: Iaee6ef8470fab24d68043b4aa72b58e9ebbd4b14