A MsgId newtype

This more strongly types the ubiquitous message id type by no longer
making it an integer.  It keeps the actual ID opaque.  Only for the
generic job API the number keeps being used.  Some locations also need
to create it from an integer and call MsgId::new().

src/error.rs

@@ -32,6 +32,8 @@ pub enum Error {
     FromUtf8(std::string::FromUtf8Error),
     #[fail(display = "{}", _0)]
     BlobError(#[cause] crate::blob::BlobError),
+    #[fail(display = "Invalid Message ID.")]
+    InvalidMsgId,
 }
 
 pub type Result<T> = std::result::Result<T, Error>;
@@ -102,6 +104,12 @@ impl From<crate::blob::BlobError> for Error {
     }
 }
 
+impl From<crate::message::InvalidMsgId> for Error {
+    fn from(_err: crate::message::InvalidMsgId) -> Error {
+        Error::InvalidMsgId
+    }
+}
+
 #[macro_export]
 macro_rules! bail {
     ($e:expr) => {