[renderer] Err on bad file names, instead of panic

Addressing the review comments.

build.rs

@@ -25,8 +25,6 @@ mod execs {
 }
 
 
-// TODO: Drop after error_chain is fixed
-#[allow(unused_doc_comment)]
 error_chain!{
  foreign_links {
         Io(std::io::Error);

src/lib.rs

@@ -100,8 +100,6 @@ pub use book::BookItem;
 pub use renderer::Renderer;
 
 /// The error types used through out this crate.
-// TODO: Drop after error_chain is fixed
-#[allow(unused_doc_comment)]
 pub mod errors {
     error_chain!{
         foreign_links {

src/renderer/html_handlebars/hbs_renderer.rs

@@ -66,8 +66,9 @@ impl HtmlHandlebars {
                 let filepath = Path::new(&ch.path).with_extension("html");
                 let rendered = self.post_process(rendered,
                     &normalize_path(filepath.to_str()
-                        .expect(&format!("Bad file name: {}", filepath.display()))),
-                    ctx.book.get_html_config().get_playpen_config());
+                        .ok_or(Error::from(format!("Bad file name: {}", filepath.display())))?),
+                    ctx.book.get_html_config().get_playpen_config()
+                );
 
                 // Write to file
                 info!("[*] Creating {:?} ✓", filepath.display());
@@ -458,7 +459,7 @@ fn id_from_content(content: &str) -> String {
     let mut content = content.to_string();
 
     // Skip any tags or html-encoded stuff
-    static REPL_SUB: &[&str] = &[
+    const REPL_SUB: &[&str] = &[
         "<em>",
         "</em>",
         "<code>",