Improve error display for invalid witness version

We have two errors that cover invalid witness version, for one we print
the field element which can be confusing for some values and for the
other we omit the invalid version all together - we can do better.

Print the field element as well as the integer value when displaying the
two invalid witness version errors.

Fix: #162

src/primitives/decode.rs

@@ -602,7 +602,8 @@ impl fmt::Display for SegwitHrpstringError {
                 len,
                 segwit::MAX_STRING_LENGTH
             ),
-            InvalidWitnessVersion(fe) => write!(f, "invalid segwit witness version: {}", fe),
+            InvalidWitnessVersion(fe) =>
+                write!(f, "invalid segwit witness version: {} (fe32='{}')", fe.to_u8(), fe),
             Padding(ref e) => write_err!(f, "invalid padding on the witness data"; e),
             WitnessLength(ref e) => write_err!(f, "invalid witness length"; e),
             Checksum(ref e) => write_err!(f, "invalid checksum"; e),

src/primitives/segwit.rs

@@ -67,7 +67,7 @@ pub struct InvalidWitnessVersionError(pub Fe32);
 
 impl fmt::Display for InvalidWitnessVersionError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "field element does not represent a valid witness version")
+        write!(f, "invalid segwit witness version: {} (fe32='{}')", self.0.to_u8(), self.0)
     }
 }