error message improvements

src/error.rs

@@ -85,11 +85,11 @@ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
             EvalError::FunctionPointerTyMismatch(expected, got) =>
                 write!(f, "tried to call a function of type {:?} through a function pointer of type {:?}", expected, got),
             EvalError::ArrayIndexOutOfBounds(span, len, index) =>
-                write!(f, "array index {} out of bounds {} at {:?}", index, len, span),
+                write!(f, "index out of bounds: the len is {} but the index is {} at {:?}", len, index, span),
             EvalError::Math(span, ref err) =>
-                write!(f, "mathematical operation at {:?} failed with {:?}", span, err),
+                write!(f, "{:?} at {:?}", err, span),
             EvalError::InvalidChar(c) =>
-                write!(f, "invalid utf8 character: {}", c),
+                write!(f, "tried to interpret an invalid 32-bit value as a char: {}", c),
             _ => write!(f, "{}", self.description()),
         }
     }