Unclear "Invalid digit found in string" error for invalid process lifetimes

Process lifetimes must take integer values (i.e. whole number of years). If float values (e.g. 15.1) or negative values (e.g. -15) are provided, there should be a clear error message saying that only positive integer values are allowed. Instead, we currently get a rather cryptic "Invalid digit found in string" message. Presumably this comes from serde trying to deserialise into a `u32`. Perhaps instead we need to deserialise into a float, do the validation checks manually, then convert to `u32` (or some other approach).

Side note, but the tests are currently only checking for the presence of an error, not the exact error message. We should fix this up as well.

Locate the process-lifetime deserialization and its existing tests, then run those tests to reproduce the cryptic error for float and negative values. Make invalid values produce a clear message that only positive integers are allowed, and update the tests to assert the exact message.

Written by the indexing model from the issue text.