Fix expected error codes for compressed texture updates

[lexaknyazev]

Either error code is acceptable because:

When a command could potentially generate several different errors ..., the GL implementation may choose to generate any of the applicable errors.

Generic rule:

If a command that requires an enumerated value is passed a symbolic constant that is not one of those specified as allowable for that command, an INVALID_ENUM error is generated.

Command-specific rule:

An INVALID_OPERATION error is generated if format does not match the internal format of the texture image being modified...

[kenrussell]

Looks good to me!