Add backtrace of error statements

That change gives backtrace of errors that should simplify
deducing what the error is all about.

The following code:
```cpp
f: () = {
  l1 := :() -> int = {
    i := 42;;
    return i;
  };
}
```
Generates error:
```
bug_352_error_message_semicolon.cpp2...
bug_352_error_message_semicolon.cpp2(3,13): error: empty statement is not allowed - remove extra semicolon (at ';')
bug_352_error_message_semicolon.cpp2(3,13): error: bad statement! (at ';')
bug_352_error_message_semicolon.cpp2(2,6): error: bad statement! (at ':')
```

Author: filipsajdak

regression-tests/test-results/pure2-bounds-safety-pointer-arithmetic-error.cpp2.output

@@ -1,3 +1,5 @@
 pure2-bounds-safety-pointer-arithmetic-error.cpp2...
 pure2-bounds-safety-pointer-arithmetic-error.cpp2(15,13): error: 'delete' and owning raw pointers are not supported in Cpp2 - use unique.new<T>, shared.new<T>, or gc.new<T> instead (in that order)
+pure2-bounds-safety-pointer-arithmetic-error.cpp2(15,16): error: bad statement! (at 'first')
+pure2-bounds-safety-pointer-arithmetic-error.cpp2(9,25): error: bad statement! (at '{')
 

source/parse.h

@@ -4507,6 +4507,7 @@ class parser
         while (curr().type() != lexeme::RightBrace) {
             auto s = statement(true);
             if (!s) {
+                error("bad statement!");
                 pos = start_pos;    // backtrack
                 return {};
             }