Improve submodel error message

Author: penelopeysm

src/context_implementations.jl

@@ -109,11 +109,8 @@ Falls back to `tilde_observe!!(context, right, left, vi)` ignoring the informati
 and indices; if needed, these can be accessed through this function, though.
 """
 function tilde_observe!!(::DefaultContext, right, left, vn, vi)
-    right isa DynamicPPL.Submodel && throw(
-        ArgumentError(
-            "`~` with a submodel on the right-hand side of an observe statement is not supported",
-        ),
-    )
+    right isa DynamicPPL.Submodel &&
+        throw(ArgumentError("`x ~ to_submodel(...)` is not supported when `x` is observed"))
     vi = accumulate_observe!!(vi, right, left, vn)
     return left, vi
 end

src/model.jl

@@ -258,7 +258,7 @@ julia> # However, it's not possible to condition `inner` directly.
        conditioned_model_fail = model | (inner = 1.0, );
 
 julia> conditioned_model_fail()
-ERROR: ArgumentError: `~` with a model on the right-hand side of an observe statement is not supported
+ERROR: ArgumentError: `x ~ to_submodel(...)` is not supported when `x` is observed
 [...]
 ```
 """

src/submodel.jl

@@ -144,8 +144,8 @@ julia> @model illegal_likelihood() = a ~ to_submodel(inner())
 illegal_likelihood (generic function with 2 methods)
 
 julia> model = illegal_likelihood() | (a = 1.0,);
-julia> model()
-ERROR: ArgumentError: `~` with a model on the right-hand side of an observe statement is not supported
+julia> model(a)
+ERROR: ArgumentError: `x ~ to_submodel(...)` is not supported when `x` is observed
 [...]
 ```
 """