Low: libcrmcommon: improve ISO 8601 error handling

This checks more error conditions, logs better error messages, and
always returns NULL for invalid specifications.

Significantly, this drops the CRM_CHECK() assertions for invalid specifications;
the values are provided by the user, not a bug, and should be handled
appropriately. This improves error output from the iso8601 tool.

As a side effect, this fixes several edge case bugs.