Support try/except ImportError

[rchen152]

The following is a common code pattern:

try:
  import foo
except ImportError:
  foo = <some fallback>

It's easy enough to add # pytype: disable=import-error on the import line, but we might be able to use the error checkpointing code to roll back import errors that occur inside a try/except block.

[dbarnett]

I had a similar problem importing the typing module:

try:
    from backports import typing
except ImportError:
    import typing

def foo() -> typing.Union[X, Y]:
    ...

There's no error from the import but it seems to not understand what typing refers to after, and gives me errors about typing.FOO claiming it could be Any:

File "myfile.py", line 6, in <module>: Invalid type annotation 'Any or Union[X, Y]' for return [invalid-annotation]
  Must be constant

If I delete the try/except then either import by itself works as expected, ~either import typing or from backports import typing~.

EDIT: Sorry, it's actually an issue about backports.typing, not the try/except. I realized from backports import typing by itself does not work as expected, it's just silently losing type information and getting typing: Any.