When presenting an error that may be a PathError, the error's path is
usually relative to some other value. If the caller is able to express
that value (or, more often, a reference to it) in HCL syntax then this
method will produce a complete expression in the error message,
concatenating any path information from the error to the end of the given
prefix string.
cty doesn't have a native string representation of a cty.Path because it
is the layer below any particular syntax, but up here in Terraform land
we know that we use HCL native syntax and so we can format a string in
a HCL-ish way for familiarity to the user.
We'll use this form automatically when such an error is used directly as a
diagnostic, but we also expose the function publicly so that other code
that incorporates errors into diagnostic detail strings can apply the
same formatting.
