BaseTools:corrected error message for the DatumType error

BaseTools:corrected error message for the DatumType error

BZ:https://bugzilla.tianocore.org/show_bug.cgi?id=1762

For structure Pcd, Its DatumType may be self-defined,
If the PCD DatumType format is incorrect,
this PCD DatumType error message is inappropriate.

This patch corrects the error message.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Zhiju.Fan <zhijux.fan@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>