make custom mir ICE a bit nicer

compiler/rustc_mir_build/src/build/custom/mod.rs

@@ -118,7 +118,11 @@ fn parse_attribute(attr: &Attribute) -> MirPhase {
                 phase = Some(value);
             }
             other => {
-                panic!("Unexpected key {}", other);
+                span_bug!(
+                    nested.span(),
+                    "Unexpected key while parsing custom_mir attribute: '{}'",
+                    other
+                );
             }
         }
     }