fix an error and reorder some functions (#650)

ext/DynamicPPLMCMCChainsExt.jl

@@ -8,12 +8,6 @@ else
     using ..MCMCChains: MCMCChains
 end
 
-_has_varname_to_symbol(info::NamedTuple{names}) where {names} = :varname_to_symbol in names
-function _check_varname_indexing(c::MCMCChains.Chains)
-    return DynamicPPL.supports_varname_indexing(c) ||
-           error("Chains do not support indexing using $vn.")
-end
-
 # Load state from a `Chains`: By convention, it is stored in `:samplerstate` metadata
 function DynamicPPL.loadstate(chain::MCMCChains.Chains)
     if !haskey(chain.info, :samplerstate)
@@ -26,10 +20,17 @@ function DynamicPPL.loadstate(chain::MCMCChains.Chains)
     return chain.info[:samplerstate]
 end
 
-# A few methods needed.
+_has_varname_to_symbol(info::NamedTuple{names}) where {names} = :varname_to_symbol in names
+
 function DynamicPPL.supports_varname_indexing(chain::MCMCChains.Chains)
     return _has_varname_to_symbol(chain.info)
 end
+
+function _check_varname_indexing(c::MCMCChains.Chains)
+    return DynamicPPL.supports_varname_indexing(c) ||
+           error("Chains do not support indexing using `VarName`s.")
+end
+
 function DynamicPPL.getindex_varname(
     c::MCMCChains.Chains, sample_idx, vn::DynamicPPL.VarName, chain_idx
 )