update error message for large classical registers (#124)

* update error message for large classical registers

* fix format

* remove linebreak

pytket/qir/conversion/api.py

@@ -77,7 +77,10 @@ def pytket_to_qir(
 
     for creg in circ.c_registers:
         if creg.size > 64:
-            raise ValueError("classical registers must not have more than 64 bits")
+            raise ValueError(
+                """classical registers must not have more than 64 bits, \
+you could try to set cut_pytket_register=True in the conversion"""
+            )
 
     m = tketqirModule(
         name=name,