Import errors due to unexpected format are virtually
always due to version skew. Don't panic but report a
good error message (incl. hint that the imported package
needs to be reinstalled) if not in debugFormat mode.

Recognize export data format version and store it so
it can be used to automatically handle minor version
differences. We did this before, but not very well.

No export data format changes.

Manually tested with corrupted export data.

For #16881.

Change-Id: I53ba98ef747b1c81033a914bb61ee52991f35a90
Reviewed-on: https://go-review.googlesource.com/27814Reviewed-by: Matthew Dempsky <mdempsky@google.com>