An example problem:

 set a 1
 set b {}  ;# <---- this represents bad data from elsewhere

 expr {$a+$b}
 can't use empty string as operand of "+"

Which doesn't tell us which part of the expression was bad

By replacing the [expr] command we can catch the error and try harder to
help by showing the entire expression. 9ofetn complex expressions will have
many "+" (as the example) operators and we don't know which had a bad operand.

======
set _debugExpr_ 1

if {[info exists _debugExpr_]} {
  rename expr _expr
  proc expr {args} {
    if {[catch {set result [uplevel _expr $args]}]} {
      error [uplevel list "error in expression: '" {*}$args "'"]
    }
    set result
  }
}

% expr {$a+$b}
error in expression: ' {$a+$b} '

Beware, this comes at the price of speed:
% time {expr $a/.5} 1000
25.975 microseconds per iteration
% time {_expr $a/.5} 1000
17.065 microseconds per iteration
% time {expr {$a/.5}} 1000
15.988 microseconds per iteration
% time {_expr {$a/.5}} 1000
0.709 microseconds per iteration
======

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