kernel_loader: implement error trait

This implements the error trait so we can get more useful error
information back on failures.

BUG=chromium:797868
TEST=./build_test passes on all architectures
TEST=crosvm runs on caroline

Change-Id: I888cbe6fca8bc3885eeb4ff747f7b9dc51e0b587
Reviewed-on: https://chromium-review.googlesource.com/938311
Commit-Ready: Sonny Rao <sonnyrao@chromium.org>
Tested-by: Sonny Rao <sonnyrao@chromium.org>
Reviewed-by: Sonny Rao <sonnyrao@chromium.org>

kernel_loader/src/lib.rs

@@ -7,6 +7,8 @@ extern crate sys_util;
 use std::mem;
 use std::ffi::CStr;
 use std::io::{Read, Seek, SeekFrom};
+use std::error::{self, Error as KernelLoaderError};
+use std::fmt::{self, Display};
 
 use sys_util::{GuestAddress, GuestMemory};
 
@@ -34,6 +36,45 @@ pub enum Error {
 }
 pub type Result<T> = std::result::Result<T, Error>;
 
+impl error::Error for Error {
+    fn description(&self) -> &str {
+        match self {
+            &Error::BigEndianElfOnLittle =>
+                "Trying to load big-endian binary on little-endian machine",
+            &Error::CommandLineCopy =>
+                "Failed writing command line to guest memory",
+            &Error::CommandLineOverflow =>
+                "Command line overflowed guest memory",
+            &Error::InvalidElfMagicNumber =>
+                "Invalid Elf magic number",
+            &Error::InvalidProgramHeaderSize =>
+                "Invalid program header size",
+            &Error::InvalidProgramHeaderOffset =>
+                "Invalid program header offset",
+            &Error::InvalidProgramHeaderAddress =>
+                "Invalid Program Header Address",
+            &Error::ReadElfHeader =>
+                "Unable to read elf header",
+            &Error::ReadKernelImage =>
+                "Unable to read kernel image",
+            &Error::ReadProgramHeader =>
+                "Unable to read program header",
+            &Error::SeekKernelStart =>
+                "Unable to seek to kernel start",
+            &Error::SeekElfStart =>
+                "Unable to seek to elf start",
+            &Error::SeekProgramHeader =>
+                "Unable to seek to program header",
+        }
+    }
+}
+
+impl Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "Kernel Loader Error: {}", Error::description(self))
+    }
+}
+
 /// Loads a kernel from a vmlinux elf image to a slice
 ///
 /// # Arguments