wiggle GuestError: improve Display of InFunc, InDataField

crates/wiggle/src/error.rs

@@ -19,14 +19,15 @@ pub enum GuestError {
     BorrowCheckerOutOfHandles,
     #[error("Slice length mismatch")]
     SliceLengthsDiffer,
-    #[error("In func {funcname}:{location}:")]
+    #[error("In func {modulename}::{funcname} at {location}: {err}")]
     InFunc {
+        modulename: &'static str,
         funcname: &'static str,
         location: &'static str,
         #[source]
         err: Box<GuestError>,
     },
-    #[error("In data {typename}.{field}:")]
+    #[error("In data {typename}.{field}: {err}")]
     InDataField {
         typename: String,
         field: String,