auto merge of #20802 : huonw/rust/book-css, r=steveklabnik

There is likely to be new users with the alpha release, and there are a lot of documents on the internet (StackOverflow, reddit, blogs) that refer to these guides, so emitting a more helpful error than "404" is nice. Hence, I've temporarily reinstated stub documents for each of the old guides, referring to as relevant a part of the book as possible.

Also, rustbook was silently ignoring some errors, which lead to an inconsistency with directory creation/file writing. This meant the CSS file was not being written if no `doc` directory existed in the users build dir (e.g. the buildbots). This should mean that the CSS will appear automatically in later builds.

src/doc/guide-crates.md

@@ -0,0 +1,4 @@
+% The (old) Rust Crates and Modules Guide
+
+This content has moved into the
+[the Rust Programming Language book](book/crates-and-modules.html).

src/doc/guide-error-handling.md

@@ -0,0 +1,4 @@
+% Error Handling in Rust
+
+This content has moved into the
+[the Rust Programming Language book](book/error-handling.html).

src/doc/guide-ffi.md

@@ -0,0 +1,4 @@
+% The (old) Rust Foreign Function Interface Guide
+
+This content has moved into the
+[the Rust Programming Language book](book/ffi.html).

src/doc/guide-macros.md

@@ -0,0 +1,4 @@
+% The (old) Rust Macros Guide
+
+This content has moved into the
+[the Rust Programming Language book](book/macros.html).

src/doc/guide-ownership.md

@@ -0,0 +1,4 @@
+% The (old) Rust Ownership Guide
+
+This content has moved into the
+[the Rust Programming Language book](book/ownership.html).

src/doc/guide-plugins.md

@@ -0,0 +1,4 @@
+% The (old) Rust Compiler Plugins Guide
+
+This content has moved into the
+[the Rust Programming Language book](book/plugins.html).

src/doc/guide-pointers.md

@@ -0,0 +1,4 @@
+% The (old) Rust Pointer Guide
+
+This content has moved into the
+[the Rust Programming Language book](book/pointers.html).

src/doc/guide-strings.md

@@ -0,0 +1,4 @@
+% The (old) Guide to Rust Strings
+
+This content has moved into the
+[the Rust Programming Language book](book/strings.html).

src/doc/guide-tasks.md

@@ -0,0 +1,4 @@
+% The (old) Rust Threads and Communication Guide
+
+This content has moved into the
+[the Rust Programming Language book](book/tasks.html).

src/doc/guide-testing.md

@@ -0,0 +1,4 @@
+% The (old) Rust Testing Guide
+
+This content has moved into the
+[the Rust Programming Language book](book/testing.html).

src/doc/guide-unsafe.md

@@ -0,0 +1,4 @@
+% Writing Safe Low-level and Unsafe Code in Rust
+
+This content has moved into the
+[the Rust Programming Language book](book/unsafe.html).

src/doc/guide.md

@@ -0,0 +1,4 @@
+% The (old) Rust Guide
+
+This content has moved into the
+[the Rust Programming Language book](book/README.html).