display more info when .git_store_meta is malformatted

[samirnoman]

I would like to thank you for this valuable tool which bridges an important gap in git.

When using git-store-meta.pl, I sometimes encounter the error:

.git_store_meta is malformatted.
Fix it or run --store to create new

And by inspecting the file it's not clear why!
These errors seem like random (I haven't been able to find what causes them)!

It will very helpful to display more information to aid in diagnosing this error such as invalid version number or invalid field or missing field, ... etc.

So, I made an attempt at making this with this PR (#45 Add error info to get_cache_header_info and display it to end user) to return a meaningful error from the function get_cache_header_info and display it. This should help diagnose these errors in the future in order to be able to solve them quickly