report the context, if possible, on python tracebacks

The interpreter tries to catch any exception and add the latest node
information to it, but currently we only used that to print better
formatted error messages on MesonException.

Since we should theoretically have that property for most/all
exceptions, let's percolate that upward, and message the user that an
unexpected traceback was encountered, that it should be reported as a
bug, and the helpful information of "how far into parsing this
meson.build did we get before erroring out, anyway?"

mesonbuild/mesonmain.py

@@ -27,7 +27,7 @@
 from . import mesonlib
 from . import mlog
 from . import mconf, mdist, minit, minstall, mintro, msetup, mtest, rewriter, msubprojects, munstable_coredata, mcompile, mdevenv
-from .mesonlib import MesonException
+from .mesonlib import MesonException, MesonBugException
 from .environment import detect_msys2_arch
 from .wrap import wraptool
 
@@ -144,10 +144,16 @@ def run(self, args):
             if os.environ.get('MESON_FORCE_BACKTRACE'):
                 raise
             return 1
-        except Exception:
+        except Exception as e:
             if os.environ.get('MESON_FORCE_BACKTRACE'):
                 raise
             traceback.print_exc()
+            msg = 'Unhandled python exception'
+            if all(getattr(e, a, None) is not None for a in ['file', 'lineno', 'colno']):
+                e = MesonBugException(msg, e.file, e.lineno, e.colno) # type: ignore
+            else:
+                e = MesonBugException(msg)
+            mlog.exception(e)
             return 2
         finally:
             mlog.shutdown()