Compliant error classes

Author: rhpvorderman

src/isal/isal_zlib.pyx

@@ -64,9 +64,12 @@ Z_SYNC_FLUSH=ISAL_SYNC_FLUSH
 Z_FULL_FLUSH=ISAL_FULL_FLUSH
 Z_FINISH=ISAL_FULL_FLUSH
 
-class IsalError(Exception):
+class IsalError(OSError):
     pass
 
+# Add error for compatibility
+error = IsalError
+
 
 if ISAL_DEF_MAX_HIST_BITS > zlib.MAX_WBITS:
     raise  IsalError("ISAL max window size no longer compatible with zlib. "