$($a$)$ Describe briefly what an exception is$,$ and explain the usage of the try, except and finally statements. $($b$)$ The following program asks the user for a temperature in degrees Fahrenheit, and prints the temperature in degrees Celsius instead. def f$2$c$($x$)$: return $5*($x$-32)/9$ s $=$ input$("$Enter a temperature in F:$")$ f $=$ float$($s$)$ print$("\{\}$F is $\{\}$C$".$format$($f$,$ f$2$c$($f$)))$ This program crashes if the user enters a string that cannot be converted to a float. Rewrite the program using try and except so that it repeatedly asks the user for a string, until a string that can be converted to a float is given, before printing the result.