imservice: Return something more resembling an Error on failure

The error type is expected to be printable by logging utilities.

src/imservice.rs

@@ -1,5 +1,6 @@
 use std::boxed::Box;
 use std::ffi::CString;
+use std::fmt;
 use std::num::Wrapping;
 use std::string::String;
 
@@ -246,10 +247,17 @@ pub enum ContentPurpose {
     Terminal = 13,
 }
 
+// Utilities from ::logging need a printable error type
+pub struct UnrecognizedValue;
+
+impl fmt::Display for UnrecognizedValue {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Unrecognized value")
+    }
+}
+
 impl TryFrom<u32> for ContentPurpose {
-    // There's only one way to fail: number not in protocol,
+    type Error = UnrecognizedValue;
-    // so no special error type is needed
-    type Error = ();
     fn try_from(num: u32) -> Result<Self, Self::Error> {
         use self::ContentPurpose::*;
         match num {
@@ -267,7 +275,7 @@ impl TryFrom<u32> for ContentPurpose {
             11 => Ok(Time),
             12 => Ok(Datetime),
             13 => Ok(Terminal),
-            _ => Err(()),
+            _ => Err(UnrecognizedValue),
         }
     }
 }
@@ -280,14 +288,12 @@ pub enum ChangeCause {
 }
 
 impl TryFrom<u32> for ChangeCause {
-    // There's only one way to fail: number not in protocol,
+    type Error = UnrecognizedValue;
-    // so no special error type is needed
-    type Error = ();
     fn try_from(num: u32) -> Result<Self, Self::Error> {
         match num {
             0 => Ok(ChangeCause::InputMethod),
             1 => Ok(ChangeCause::Other),
-            _ => Err(())
+            _ => Err(UnrecognizedValue)
         }
     }
 }