Syncing when the inbox buffer is read-only leads to unhelpful error:inbox-notfound

[nick4f42]

If org-caldav-inbox is opened as a buffer with buffer-read-only enabled, org-caldav-sync fails with:

== Sync errors: 

UID: xxxxx
   Title: (Could not find UID)
   Status: error  Action: error:inbox-notfound

It requires looking through the source to find the true issue that the buffer is read-only.

Since I use (setopt org-caldav-sync-direction 'cal->org), I set my inbox to read-only with a dir-local variable so I don't accidentally make changes. I would prefer org-caldav to inhibit read-only mode rather than erroring, but at least a helpful error message would be nice.

Thanks for your work on this package.