Refactor a bit

Rename the Image* types to Tiff*
Extract the "common" types to the top level.

src/error.rs

@@ -0,0 +1,53 @@
+use std::error::Error;
+use std::fmt;
+use std::io;
+
+/// An enumeration of Tiff errors
+#[derive(Debug)]
+pub enum TiffError {
+    /// The Image is not formatted properly
+    FormatError(String),
+
+    /// The Decoder does not support this image format
+    UnsupportedError(String),
+
+    /// An I/O Error occurred while decoding the image
+    IoError(io::Error),
+}
+
+impl fmt::Display for TiffError {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        match *self {
+            TiffError::FormatError(ref e) => write!(fmt, "Format error: {}", e),
+            TiffError::UnsupportedError(ref f) => write!(fmt, "The Decoder does not support the \
+                                                                 image format `{}`", f),
+            TiffError::IoError(ref e) => e.fmt(fmt),
+        }
+    }
+}
+
+impl Error for TiffError {
+    fn description (&self) -> &str {
+        match *self {
+            TiffError::FormatError(..) => "Format error",
+            TiffError::UnsupportedError(..) => "Unsupported error",
+            TiffError::IoError(..) => "IO error",
+        }
+    }
+
+    fn cause (&self) -> Option<&Error> {
+        match *self {
+            TiffError::IoError(ref e) => Some(e),
+            _ => None
+        }
+    }
+}
+
+impl From<io::Error> for TiffError {
+    fn from(err: io::Error) -> TiffError {
+        TiffError::IoError(err)
+    }
+}
+
+/// Result of an image decoding/encoding process
+pub type TiffResult<T> = Result<T, TiffError>;