errors: Clarify message for MirNodeToDataflowFailed

I've always been a little confused by the "couldn't be made" label here
- I think "couldn't be lowered to dataflow" is a little bit clearer.

Change-Id: I90de3681838933a3d8d888c93802a3f6fb0e8002
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/5970
Tested-by: Buildkite CI
Reviewed-by: Ethan Donowitz <[email protected]>

Author: glittershark

readyset-errors/src/lib.rs

@@ -177,7 +177,7 @@ pub enum ReadySetError {
         source: Box<ReadySetError>,
     },
 
-    #[error("MIR node '{index}' couldn't be made: {source}")]
+    #[error("MIR node '{index}' couldn't be lowered to dataflow: {source}")]
     MirNodeToDataflowFailed {
         /// The index (within the MirGraph) of the MIR node that couldn't
         /// be made.