Adds an error message to the {@link DataMap}. If a value is not mapped to {@link #ERROR_KEY}, bind the specified error message to {@link #ERROR_KEY}. Otherwise, replace the value of {@link #ERROR_KEY} witha new {@link String} constructed by appending the specified error message tothe previous value of {@link #ERROR_KEY}.
@param msg provides the error message to add.
@return the new value of {@link #ERROR_KEY}.
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