Improve error message for infer_setter_property

If the setter doesn't start with `set_*` then we currently panic, but
panicking is bad! Instead let's thread through structured errors to make
sure they make their way to the top

crates/macro/ui-tests/invalid-setter.rs

@@ -0,0 +1,19 @@
+extern crate wasm_bindgen;
+
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+extern "C" {
+    type A;
+
+    #[wasm_bindgen(setter, method)]
+    fn a(this: &A, b: i32);
+
+    #[wasm_bindgen(setter = x, method)]
+    fn b(this: &A, b: i32);
+
+    #[wasm_bindgen(setter, method, js_name = x)]
+    fn c(this: &A, b: i32);
+}
+
+fn main() {}

crates/macro/ui-tests/invalid-setter.stderr

@@ -0,0 +1,14 @@
+error: setters must start with `set_`, found: a
+  --> $DIR/invalid-setter.rs:10:8
+   |
+10 |     fn a(this: &A, b: i32);
+   |        ^
+
+error: setters must start with `set_`, found: x
+  --> $DIR/invalid-setter.rs:15:46
+   |
+15 |     #[wasm_bindgen(setter, method, js_name = x)]
+   |                                              ^
+
+error: aborting due to 2 previous errors
+