Add error code for a non empty directory, make

This commit creates an error code for a non-empty directory, makes
Common::COMMON_UNKNOWN_ERROR have it's error message defined outside the
error message table (it's still referenced in the table, but the change
is part one of a bug fix.) and updates the error message for
Common::FILEUTILLS_EMPTY_DIRECTORY to be consistant with the wording of
other error messages in the table.
Actually return a valid message when given an unkn

This commit is the final part of a bug fix started with the last commit.
This commit makes Common::Get_Error_Message() return the error message
for Common::COMMON_UNKNOWN_ERROR by default if the given error code does
not exist in the error code table or if the given error code does not have a error message defined for it in the error message table instead of returning NULL.
Update function description for Get_Error_Message.

This commit updates the function description for
Common::Get_Error_Message() to reflect the bug fix change make by the
last commit. (The function will return the error message for
Common::COMMON_UNKNOWN_ERROR by default.)