fixed the error message for a user to open the crate (#14969)

### What does this PR try to resolve?

This PR is resolving an
[issue](#14776 (comment))
where `cargo doc --open --examples` does not give a clear message when
the examples file is given to it.

I changed the message from `no crates with documentation` in the
[cargo_doc.rs](https://github.com/rust-lang/cargo/blob/58b2d609ece6bd0333e4d0f63a024fe3dd350b4b/src/cargo/ops/cargo_doc.rs#L62)
and
[doc.rs](https://github.com/rust-lang/cargo/blob/58b2d609ece6bd0333e4d0f63a024fe3dd350b4b/tests/testsuite/doc.rs#L1504)
to `requested crate documentation is not available to open`. Now it
becomes easy for the user to understand what they're missing.

 Fixes #14776

### How should we test and review this PR?

Here is the command through which you can test this change:

`cargo test SNAPSHOTS=overwrite -- doc::open_no_doc_crate`

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src/cargo/ops/cargo_doc.rs

@@ -56,10 +56,11 @@ pub fn doc(ws: &Workspace<'_>, options: &DocOptions) -> CargoResult<()> {
     let compilation = ops::compile(ws, &options.compile_opts)?;
 
     if options.open_result {
-        let name = &compilation
-            .root_crate_names
-            .get(0)
-            .ok_or_else(|| anyhow::anyhow!("no crates with documentation"))?;
+        let name = &compilation.root_crate_names.get(0).ok_or_else(|| {
+            anyhow::anyhow!(
+                "cannot open specified crate's documentation: no documentation generated"
+            )
+        })?;
         let kind = options.compile_opts.build_config.single_requested_kind()?;
 
         let path = path_by_output_format(&compilation, &kind, &name, &options.output_format);

tests/testsuite/doc.rs

@@ -1501,7 +1501,7 @@ fn open_no_doc_crate() {
         .with_status(101)
         .with_stderr_data(str![[r#"
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
-[ERROR] no crates with documentation
+[ERROR] cannot open specified crate's documentation: no documentation generated
 
 "#]])
         .run();