[no ci] Improve emcee error message

Author: penelopeysm

src/mcmc/emcee.jl

@@ -50,8 +50,11 @@ function AbstractMCMC.step(
 
     # Update the parameters if provided.
     if initial_params !== nothing
-        if !(initial_params isa AbstractVector && length(initial_params) == n)
-            err_msg = "initial_params for `Emcee` must be a vector of initialisation strategies, with length equal to the number of walkers ($n)"
+        if !(
+            initial_params isa AbstractVector{<:DynamicPPL.AbstractInitStrategy} &&
+            length(initial_params) == n
+        )
+            err_msg = "initial_params for `Emcee` must be a vector of `DynamicPPL.AbstractInitStrategy`, with length equal to the number of walkers ($n)"
             throw(ArgumentError(err_msg))
         end
         vis = map(vis, initial_params) do vi, strategy