fix catching polymorphic types by value

Compiling with GCC 8 produces a new warning about catching polymorphic
types by value. Proper way is to use references instead.

/home/sharkcz/projects/pulseview/main.cpp: In function ‘int main(int, char**)’:
/home/sharkcz/projects/pulseview/main.cpp:229:22: error: catching polymorphic type ‘class std::exception’ by value [-Werror=catch-value=]
   } catch (exception e) {
                      ^
cc1plus: all warnings being treated as errors