When a CRX is added to the Chrome web store, Google adds a directory _metadata to the crx. Older versions of Chrome do not know about this directory (tested on Chrome 27) and throw this error:
Package is invalid. Details: 'Cannot load extension with file or directory name
_metadata. Filenames starting with "_" are reserved for use by the system.'.
You can see this error if you try to install this extension on an older version of Chrome: https://chrome.google.com/webstore/detail/allcast-receiver/hjbljnpdahefgnopeohlaeohgkiidnoe
A good explanation: https://groups.google.com/a/chromium.org/d/msg/chromium-apps/huDqSeaQx3Q/Z5GOMxA7lQIJ
Is there a workaround for this? This makes it impossible to support older versions of Chrome.
You -as an extension developer- cannot work-around this bug, because the directory is added by the Chrome Web Store (CWS). You could work-around the bug by not using the CWS to distribute the extension, but then the extension cannot be used by the majority of Chrome users, because Google has actively tried to stop users from getting extensions from outside the CWS.
The CWS team are the only ones who can fix the bug. They should serve crx files without a _metadata directory if the user agent does not support the directory (Chrome 30-). This bug is being tracked on https://code.google.com/p/chromium/issues/detail?id=377278. So far, the response is:
We can't fix old versions of Chrome. Webstore side tracked at b/15176392, but extending support back to Chrome 30 is unlikely.
Star the issue to get notified of updates and to signal that you really want the bug to be fixed. If enough users/developers complain, then Google might fix the bug.