Demote "no results" error to warning in elink_list method

[allenbaron]

For a single object, a lack of API results should (and current does) result in an error:
https://github.com/allenbaron/DO.utils/blob/0e29fb1d8716385d54beea020046a4830b2479c0/R/extract.R#L83-L90

However, this error is propagated to the extract_pmid.elink_list() method, which can cause issues if a new publication is included in the list and has not yet been cited. A parameter needs to be added to control whether this error should occur or if it should be demoted to a warning.

Handling is also needed to make the warning less repetitive/verbose and more informative.