Most of the errors in this package are holdovers from our previous
syetem where we had our own custom compiler and evaluator and are no
longer needed. The few we still use during plan applicaton (via the
diagnostics system, which is another component from the old system
that we still use) have been promoted into the diag package. Doing so,
allows us to not have to import "github.com/pkg/errors" as "goerr" in
some parts of the engine, a nice cleaup.
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