Errors are often swallowed when calling external packages

[luxaritas]

There's two aspects of this issue:

1. In some cases (namely in pk_predictors) a string of xs is returned when the underling package/the popen call returns an error. This is arguably undesirable as it means the consumer may not be aware of a failure and so will handle it improperly (eg assuming it is all unpaired rather than invalid)
2. In many cases (whether or not an error is thrown), arnie may not output enough information to actually diagnose the underlying issue - eg by not outputing the subprocess stdout/stderr at all, or not properly converting it to a string