way to check if the evaluation of an expression returns an error?

Is there a recommended or "good" way to check if the evaluation of an expression returns an error?
   
  e.g. 
var(NA)    
  I wish  var(NA) to return NA or "err", or some other value, even a text-string, but not an error message.
   
   
  I am using a loop to load many samples of data and to perform analysis on each sample.  On occasion, the function I execute returns an error message.  (e.g. when all data is NA, etc).   I wish for the loop to continue to execute, and to have the function return an NA or NULL rather then an error message.
   

       
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