Need more info in log

[MarkSwanson]

capnpc-rust was calling panic!(), but I couldn't see what file it was trying to open.
I think this patch will help others.

$ git diff
diff --git a/src/codegen.rs b/src/codegen.rs
index 16c9be7..93c8f11 100644
--- a/src/codegen.rs
+++ b/src/codegen.rs
@@ -1596,7 +1596,7 @@ pub fn main(mut inp : T) -> ::std::io::Result<()>
use std::io::Write;
try!(writer.write_all(text.as_bytes()));
}

•        Err(e) => {panic!("could not open file for writing: {}", e)}
  

•        Err(e) => {panic!("could not open file {:?} for writing: {}", filepath, e)}
   }
  

  }
  Ok(())

[dwrensha]

Good point! Fixed in 8372056.