You can not tailor the message emitted by the Katalon’s built-in keywords.
The following URL shows the source of WS.verifyElementPropertyValue() keyword. If you read the source, you would find that the keyword does not allow users to customize the messages.
What can you do? — you can create your own version of custom keyword which does the same but with different message.
You can create your own customKeyword as describe in the doc. You can copy the source of builtlin keyword into a groovy file (with its unique package of your own, different from the original, of course). And you can modify the source line #59-60.
if (!isEqual) {
KeywordMain.stepFailed(MessageFormat.format(StringConstants.KW_LOG_FAILED_ELEMENT_PROP_VAL_NOT_EQUAL, [value, retValue.toString()] as Object[]), flowControl, null, null)
} else {
logger.logPassed(StringConstants.KW_LOG_PASSED_VERIFY_ELEMENT_PROPERTY_VALUE)
}
as you like. Then you will get your own tailored VerifyElementPropertyValueKeyword class with verifyElementPropertyValue() method.