Produce a more useful error message when rustfmt can't be found. Fixes …

…#1205

Prior to this change bindgen would simply print any error that occurred
while attempting to run rustfmt straight to stderr using fmt::Debug.
Combined with the fact that rustfmt is enabled by default now this meant that
if rustfmt was missing or not working a cryptic error would be printed.

src/lib.rs

@@ -1835,7 +1835,7 @@ impl Bindings {
                 writer.write(rustfmt_bindings.as_bytes())?;
             },
             Err(err) => {
-                eprintln!("{:?}", err);
+                eprintln!("Failed to run rustfmt: {} (non-fatal, continuing)", err);
                 writer.write(bindings.as_bytes())?;
             },
         }