Revert "better error reporting on mmap failure (#3)"

This reverts commit e9cae97.

Author: apfitzge

src/error.rs

@@ -6,5 +6,5 @@ pub enum Error {
     InvalidFileSize,
     InvalidHeader,
     IoError(std::io::Error),
-    MMapError(std::io::Error),
+    MMapError(usize),
 }

src/init.rs

@@ -120,7 +120,7 @@ fn open_mmap(file: &File, size: usize) -> Result<*mut c_void, Error> {
     };
 
     if mmap == libc::MAP_FAILED {
-        return Err(Error::MMapError(std::io::Error::last_os_error()));
+        return Err(Error::MMapError(mmap as usize));
     }
 
     Ok(mmap)