Bug 125327 [Dialogs] Error message shown in the InputDialog is not fully visible
When an error message returned by InputValidator is long, InputDialog displays it partially. My suggestion is to create error label with SWT.WRAP flag, or something like this.
bundles/org.eclipse.jface/src/org/eclipse/jface/dialogs/InputDialog.java
protected Text getText() {
        return text;
    }
    /**
     * Returns the validator.
     * 
     * @return the validator
     */
    protected IInputValidator getValidator() {
        return validator;
    }
    /**
     * Returns the string typed into this input dialog.
     * 
     * @return the input string
     */
    public String getValue() {
        return value;
    }
    /**
     * Validates the input.
     * <p>
     * The default implementation of this framework method delegates the request
     * to the supplied input validator object; if it finds the input invalid,
     * the error message is displayed in the dialog's message line. This hook
     * method is called whenever the text changes in the input field.
     * </p>
     */
    protected void validateInput() {
        String errorMessage = null;