Explicit return type with error causes type error when an error is thrown

[GregoryTravis]

(This bug refers to #10725)

This change to Decimal.add results in the error shown below, in the case of a thrown error:

-    add self (that : Decimal) (math_context : Math_Context | Nothing = Nothing) =
+    add self (that : Decimal) (math_context : Math_Context | Nothing = Nothing) -> Decimal ! Arithmetic_Error | Illegal_Argument =

Error:

    - [FAILED] should not allow mixed arithmetic (float) [68ms]
        Reason: An unexpected panic was thrown: Type error: expected the result of `add` to be Decimal, but got <ERR>.
        at <enso> Decimal.add(/Users/gmt/dev/enso/enso/built-distribution/enso-engine-0.0.0-dev-macos-aarch64/enso-0.0.0-dev/lib/Standard/Base/0.0.0-dev/src/Data/Decimal.enso:478-482)

[JaroslavTulach]

Ok. Caused by an attempt to attach warning to DataFlowError.