trigger Contact_details_on_task on Task (before insert) {
list<task> t=[select whoid,description from task];
string iid;
    set<id> s=new set<id>();
    set<id> whoid=new set<id>();
    for(task t1:t){
        s.add(t1.whoid);
    }
    for(task t3:trigger.new){
       iid =t3.whoid;

        }

    for(contact a2:[select id from contact where id=: iid]){
        whoid.add(a2.id);
    }
    list<contact> a=[select phone,name,Email,Mailingcity,Mailingstate,Mailingcountry,Mailingstreet,MailingCountryCode from contact where id in : whoid];
    system.debug(a);
    for(contact a1:a){
        for(task t2:trigger.new){
        string descr = t2.description == null ? 'blank' : t2.description;
        t2.description=descr +' rn'+a1.phone + ' rn'+a1.name+ ' rn'+a1.Mailingcity+ ' rn'+a1.Mailingstate+ ' rn'+a1.Mailingstreet+' rn'+a1.Mailingcountry+  'rn'+a1.MailingCountryCode +'rn'+a1.email;
        }}
    update t;
}

for(task t2:trigger.new){
        string descr = t2.description == null ? 'blank' : t2.description;
        t2.description=descr +' rn'+a1.phone + ' rn'+a1.name+ ' rn'+a1.Mailingcity+ ' rn'+a1.Mailingstate+ ' rn'+a1.Mailingstreet+' rn'+a1.Mailingcountry+  'rn'+a1.MailingCountryCode +'rn'+a1.email;
        }}
   // update t;

try {
    //your code here
} catch(Exception e) {
    System.debug('Exception type caught: ' + e.getTypeName());
    System.debug('Message: ' + e.getMessage());
    System.debug('Cause: ' + e.getCause());    // returns null
    System.debug('Line number: ' + e.getLineNumber());
    System.debug('Stack trace: ' + e.getStackTraceString());
}