clarify error messages

The old error messages displayed valid values as unicode strings, but the displayed actual value was in bytes. The new error messages display the valid values as bytes now to avoid confusion.

blendparse.py

@@ -167,7 +167,7 @@ class Blendfile(io.FileIO):
         else:
             raise BlendDecodeError(
                 f"Invalid pointer size character {header.pointer_size}; " \
-                "must be '-' or '_'!")
+                f"must be {b'-'} or {b'_'}!")
         if header.endianness == bytes("v", "utf-8"):
             self.endianness = "little"
         elif header.endianess == bytes("V", "utf-8"):
@@ -175,7 +175,7 @@ class Blendfile(io.FileIO):
         else:
             raise BlendDecodeError(
                 f"Invalid endianness character {header.endianess}; "\
-                "must be 'v' or 'V'")
+                f"must be {b'v'} or {b'V'}")
         if not header.version.isdigit():
             raise BlendDecodeError(
                 f"Invalid version string {header.version}!")