A better solution, and better practice in general, is for the code in the catch block to check the identifier of the error. If it can't handle it properly, it should rethrow the error, so it is reported correctly. In your case the catch block could check err.identifier to see that it was really thrown because of an invalid input path.
Also, since you are only expecting to deal with errors that come from importdata, it might be better to only have that statement in the try block:
try
data = importdata(...
catch
...
end
if isstruct ...