Error handling for filter_dob function

[steven-buckley]

At the moment the filter_dob applies some logic to limit the joins based on the number of matching characters in the dob string.

However, this is tailored towards the formatted date of birth output from the filter_dob function.

If somebody calls the matching function without applying the formatting, some warning messages may be outputted to the console.

These typically seem to occur when the dob string includes non-numeric characters and the function is trying to convert these to integers and having to switch to NA instead. This means one of the additional options for comparing dates is skipped due to lack of relevant information but the warning message is not clear that this is what has happened.

Could either handle the warning message or revise function to handle unformatted strings.