Issue in workflow

Hi,

I'm trying to track the Reason for Opportunities getting Lost. So, created a picklist field with few standard reasons shared by client alongwith a value in last i.e. Other reasons. Now, when a user selects this value from picklist; another text field should become mandatory in order to capture the descriptive reason. If not mandatory, then an error message should be displayed asking to fill that respective text field; in case user is trying to leave this field as NULL & proceed to save.

Example:

Opportunity Name - ABC

Reason Lost (This field becomes mandatory as soon as Sales Stage is changed to Closed/Lost) - "Other reasons" (This is one of the picklist values)