Give more errors for specific failure conditions

Some failure conditions implicitly fail by failing some other check.
But the error messages are more helpful if they say explicitly what
actually caused the failure, so add those as failure conditions and
errors.

test/functional/wallet_importmulti.py

@@ -552,7 +552,7 @@ class ImportMultiTest(BitcoinTestFramework):
                                "keys": [key.privkey]},
                               success=False,
                               error_code=-5,
-                              error_message="Descriptor is invalid, Missing checksum")
+                              error_message="Missing checksum")
 
         # Test importing of a P2SH-P2WPKH address via descriptor + private key
         key = get_key(self.nodes[0])