codegen: improve error messages

This is an initial step. Further improvements could offer suggestions
for what is available.

See #97.

Author: mathstuf

graphql_query_derive/src/codegen.rs

@@ -48,18 +48,26 @@ pub(crate) fn response_for_query(
         .find(|op| op.name == selected_operation)
         .map(|i| i.to_owned());
 
-    let operation = context.selected_operation.clone().unwrap_or_else(|| {
+    let opt_operation = context.selected_operation.clone().or_else(|| {
         operations
             .iter()
             .next()
             .map(|i| i.to_owned())
-            .expect("no operation in query document")
     });
+    let operation = if let Some(operation) = opt_operation {
+        operation
+    } else {
+        panic!("no operation '{}' in query document", selected_operation);
+    };
 
     let response_data_fields = {
-        let root_name: String = operation
-            .root_name(&context.schema)
-            .expect("operation type not in schema");
+        let opt_root_name = operation
+            .root_name(&context.schema);
+        let root_name: String = if let Some(root_name) = opt_root_name {
+            root_name
+        } else {
+            panic!("operation type '{:?}' not in schema", operation.operation_type);
+        };
         let definition = context
             .schema
             .objects

graphql_query_derive/src/fragments.rs

@@ -13,7 +13,12 @@ impl GqlFragment {
     pub(crate) fn to_rust(&self, context: &QueryContext) -> Result<TokenStream, ::failure::Error> {
         let derives = context.response_derives();
         let name_ident = Ident::new(&self.name, Span::call_site());
-        let object = context.schema.objects.get(&self.on).expect("oh, noes");
+        let opt_object = context.schema.objects.get(&self.on);
+        let object = if let Some(object) = opt_object {
+            object
+        } else {
+            panic!("fragment '{}' cannot operate on unknown type '{}'", self.name, self.on);
+        };
         let field_impls = object.field_impls_for_selection(context, &self.selection, &self.name)?;
         let fields = object.response_fields_for_selection(context, &self.selection, &self.name)?;