Add support for ASCII tags

src/error.rs

@@ -1,6 +1,7 @@
 use std::error::Error;
 use std::fmt;
 use std::io;
+use std::string;
 
 /// Tiff error kinds.
 #[derive(Debug)]
@@ -49,5 +50,11 @@ impl From<io::Error> for TiffError {
     }
 }
 
+impl From<string::FromUtf8Error> for TiffError {
+    fn from(err: string::FromUtf8Error) -> TiffError {
+        TiffError::FormatError(String::from("Image contains invalid tag."))
+    }
+}
+
 /// Result of an image decoding/encoding process
 pub type TiffResult<T> = Result<T, TiffError>;