parsers: use built-in string quoting instead of manual quoting

There is at least one branch of the FromStr implementation which passes
user input right into the error struct, so we cannot assume that `'` is
not part of that value.

parsers/src/data_forms/validate.rs

@@ -108,13 +108,13 @@ impl Display for DatatypeError {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
         match self {
             DatatypeError::MissingPrefix { input } => {
-                write!(f, "Missing prefix in validation datatype '{input}'.")
+                write!(f, "Missing prefix in validation datatype {input:?}.")
             }
             DatatypeError::InvalidType { input } => {
-                write!(f, "Invalid validation datatype '{input}'.")
+                write!(f, "Invalid validation datatype {input:?}.")
             }
             DatatypeError::UnknownType { input } => {
-                write!(f, "Unknown validation datatype '{input}'.")
+                write!(f, "Unknown validation datatype {input:?}.")
             }
         }
     }