bpo-35931: Gracefully handle SyntaxError in pdb debug command (GH-11782)

Previously, `debug print(` would cause the interpreter to exit on a SyntaxError whereas `print(` would properly display the error and return to the pdb prompt.

This patch fixes this by pre-compiling the code before passing it to `Pdb.run`.


https://bugs.python.org/issue35931