Be smarter about error handling in run().

`run()` returns `Result<(), String>`. But on failure it always returns
an empty string, and then `wrap_return()` treats an empty string
specially, by not reporting the error.

It turns out we already have the `ErrorReported` type for this sort of
behaviour. This commit changes `run()` to use it.

src/librustdoc/lib.rs

@@ -443,9 +443,7 @@ fn wrap_return(diag: &rustc_errors::Handler, res: Result<(), String>) -> MainRes
     match res {
         Ok(()) => Ok(()),
         Err(err) => {
-            if !err.is_empty() {
-                diag.struct_err(&err).emit();
-            }
+            diag.struct_err(&err).emit();
             Err(ErrorReported)
         }
     }
@@ -478,7 +476,7 @@ fn main_options(options: config::Options) -> MainResult {
 
     match (options.should_test, options.markdown_input()) {
         (true, true) => return wrap_return(&diag, markdown::test(options)),
-        (true, false) => return wrap_return(&diag, test::run(options)),
+        (true, false) => return test::run(options),
         (false, true) => {
             return wrap_return(
                 &diag,

src/librustdoc/test.rs

@@ -42,7 +42,7 @@ pub struct TestOptions {
     pub attrs: Vec<String>,
 }
 
-pub fn run(options: Options) -> Result<(), String> {
+pub fn run(options: Options) -> Result<(), ErrorReported> {
     let input = config::Input::File(options.input.clone());
 
     let invalid_codeblock_attributes_name = rustc_lint::builtin::INVALID_CODEBLOCK_ATTRIBUTES.name;
@@ -150,7 +150,7 @@ pub fn run(options: Options) -> Result<(), String> {
     });
     let tests = match tests {
         Ok(tests) => tests,
-        Err(ErrorReported) => return Err(String::new()),
+        Err(ErrorReported) => return Err(ErrorReported),
     };
 
     test_args.insert(0, "rustdoctest".to_string());