debug-on-error

debug-on-error is a variable defined in `C source code'.
Its value is

nil

Documentation:
Non-nil means enter debugger if an error is signaled.
Does not apply to errors handled by `condition-case' or those
matched by `debug-ignored-errors'.
If the value is a list, an error only means to enter the debugger
if one of its condition symbols appears in the list.
When you evaluate an expression interactively, this variable
is temporarily non-nil if `eval-expression-debug-on-error' is non-nil.
The command `toggle-debug-on-error' toggles this.
See also the variable `debug-on-quit' and `inhibit-debugger'.

You can customize this variable.