Continue executing after error

[anthonysena]

Scenario: when executing an analysis specification, it may be desirable to continue to execute each target module if one fails. An example: we have an analysis specification that runs CohortGenerator (CG), CohortMethod (CM) and PatientLevelPrediction (PLP). If the single threaded execution, the workflow may be CG -> CM -> PLP. If CM were to fail, PLP is not executed which then means we lose some execution time until a user intervenes.

Based on https://docs.ropensci.org/targets/reference/tar_target.html, it should be possible to set targets::tar_option_get("error") with the value of "continue" to achieve this behavior.

[anthonysena]

Noting that in v1.x we will no longer use targets internally but the notion of continuing the execution of a study if an error is encountered is still desirable.

[anthonysena]

Closing since 1) we're not planning to use targets moving forward (at least inside of Strategus) and 2) we're tracking this via #141.