Raise on unsupported exprs (#362)

Instead of the unfriendly "no case clause matching", this raises an
error telling the user that the literal is not supported. This was
already done by tds adapter.

lib/ecto/adapters/myxql/connection.ex

@@ -657,6 +657,10 @@ if Code.ensure_loaded?(MyXQL) do
       ["(0 + ", Float.to_string(literal), ?)]
     end
 
+    defp expr(expr, _sources, query) do
+      error!(query, "unsupported expression: #{inspect(expr)}")
+    end
+
     defp interval(count, "millisecond", sources, query) do
       ["INTERVAL (", expr(count, sources, query) | " * 1000) microsecond"]
     end

lib/ecto/adapters/postgres/connection.ex

@@ -745,6 +745,10 @@ if Code.ensure_loaded?(Postgrex) do
       [Float.to_string(literal) | "::float"]
     end
 
+    defp expr(expr, _sources, query) do
+      error!(query, "unsupported expression: #{inspect(expr)}")
+    end
+
     defp type_unless_typed(%Ecto.Query.Tagged{}, _type), do: []
     defp type_unless_typed(_, type), do: [?:, ?: | type]