test: Add tests for missing tempaltes

cc #50

sailfish-tests/integration-tests/tests/fails/template_not_found.rs

@@ -0,0 +1,12 @@
+use sailfish::TemplateOnce;
+use sailfish_macros::TemplateOnce;
+
+#[derive(TemplateOnce)]
+#[template(path = "not_exist.stpl")]
+struct NotExistTemplate {
+    var: usize
+}
+
+fn main() {
+    println!("{}", NotExistTemplate { var: 1996 }.render_once().unwrap());
+}

sailfish-tests/integration-tests/tests/fails/template_not_found.stderr

@@ -0,0 +1,26 @@
+error: Template file "not_exist.stpl" not found
+ --> $DIR/template_not_found.rs:5:19
+  |
+5 | #[template(path = "not_exist.stpl")]
+  |                   ^^^^^^^^^^^^^^^^
+
+error[E0599]: no method named `render_once` found for struct `NotExistTemplate` in the current scope
+  --> $DIR/template_not_found.rs:11:51
+   |
+6  | struct NotExistTemplate {
+   | ----------------------- method `render_once` not found for this
+...
+11 |     println!("{}", NotExistTemplate { var: 1996 }.render_once().unwrap());
+   |                                                   ^^^^^^^^^^^ method not found in `NotExistTemplate`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+   = note: the following trait defines an item `render_once`, perhaps you need to implement it:
+           candidate #1: `TemplateOnce`
+
+warning: unused import: `sailfish::TemplateOnce`
+ --> $DIR/template_not_found.rs:1:5
+  |
+1 | use sailfish::TemplateOnce;
+  |     ^^^^^^^^^^^^^^^^^^^^^^
+  |
+  = note: `#[warn(unused_imports)]` on by default