Issue #801 - Improve apply metrics and `run_safely()`

Description

This PR closes #801.

This PR

simplifies and updates apply_metrics() such that it doesn't produce a data.table warning anymore when a function fails
adds the name of the metric to the call of run_safely() to improve warnings. @jamesmbaazam you were thinking about this as well. It still doesn't produce errors, but I think it's a nice step.
makes sure that run_safely() is no longer exported
updates tests

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