Reduce 'data' use on non-payload bearing methods error to warning.

Resolves #622.

codegen/src/parser/route.rs

@@ -109,9 +109,11 @@ impl RouteParams {
         // Sanity check: `data` should only be used with payload methods.
         if let Some(ref data_param) = data {
             if !method.node.supports_payload() {
-                ecx.struct_span_err(data_param.span, "`data` route parameters \
-                        can only be used with payload supporting methods")
-                    .note(&format!("'{}' does not support payloads", method.node))
+                ecx.struct_span_warn(data_param.span, "`data` route parameter \
+                        used with non-payload-supporting method")
+                    .note(&format!("'{}' does not typically support payloads", method.node))
+                    .note("the 'format' attribute parameter will match against \
+                          the 'Accept' header")
                     .emit();
             }
         }

codegen/tests/ui/data-without-post.rs

@@ -5,3 +5,5 @@ extern crate rocket;
 
 #[get("/", data = "<something>")]
 fn get(something: rocket::Data) -> &'static str { "hi" }
+
+fn main() {  }

codegen/tests/ui/data-without-post.stderr

@@ -1,10 +1,9 @@
-error: `data` route parameters can only be used with payload supporting methods
+warning: `data` route parameter used with non-payload-supporting method
  --> $DIR/data-without-post.rs:6:12
   |
 6 | #[get("/", data = "<something>")]
   |            ^^^^^^^^^^^^^^^^^^^^
   |
-  = note: 'GET' does not support payloads
-
-error: aborting due to previous error
+  = note: 'GET' does not typically support payloads
+  = note: the 'format' attribute parameter will match against the 'Accept' header