Convenience macro for propagating errors when calling a function that returns a tensorstore::Result.

This macro generates multiple statements and should be invoked as follows:

Result<int> GetSomeResult();

TENSORSTORE_ASSIGN_OR_RETURN(int x, GetSomeResult());

An optional third argument specifies the return expression in the case of an error. A variable _ bound to the error absl::Status value is in scope within this expression. For example:

TENSORSTORE_ASSIGN_OR_RETURN(int x, GetSomeResult(),
                             _.Annotate("Context message"));