There is some existing practice in the "terraform" package of returning
a special error type ValidationError from EvalNode implementations in
order to return warnings without halting the graph walk even though a
non-nil error was returned.
This is a diagnostics-flavored version of that approach, allowing us to
avoid totally reworking the EvalNode concept around diagnostics and
retaining the ability to return non-fatal errors.
NonFatalErr is equivalent to the former terraform.ValidationError, while
ErrWithWarnings is a helper that automatically treats any errors as
fatal but returns NonFatalError if the diagnostics contains only warnings.
Currently we lean heavily on the Go error type as our primary means of
describing errors, and along with that use several more specialized
implementations of it in different spots for additional capabilities such
as multiple errors in one object, source code range references, etc.
We also have a rather ad-hoc approach of returning an array of warnings
from certain functions along with one or multiple errors.
This rather-disorganized approach makes it hard for us to present
user-facing error messages consistently. As a step towards mitigating
this, package tfdiags provides a model for user-facing error and warning
messages and helper functions for creating them from various other
error and warning types used elsewhere in Terraform.
This mechanism is intended to be used to report errors and warnings where
the audience is the Terraform user, and so it may go a few layers deep
down the call stack into codepaths like config parsing, interpolation, etc
but is primarily a UX concern. The deepest reaches of Terraform core will
continue using "error" as normal, with higher layers preparing error
messages for presentation to the user.
To avoid needing to change the interface of every function that might
generate error diagnostics, the Diagnostics type can be "smuggled" via
an error value through other APIs and then unwrapped at the other end,
though it will lose any naked warnings (without at least one error) along
the way, and so codepaths that are expected to generate warnings
(validation, primarily) should use the concrete Diagnostics type
throughout the call chain.
