Make error handling richer in Footer::is_compatible (#724)

* WIP implemented is_compatible

hide Footer::from_bytes from public consumption - only found Footer::extract
used outside the module

Add a new error type for IncompatibleIndex
add a prototypical call to footer.is_compatible() in ManagedDirectory::open_read
to make sure we error before reading it further

* Make error handling more ergonomic

Add an error subtype for OpenReadError and converters to TantivyError

* Remove an unnecessary assert

it's follower by the same check that Errors instead of panicking

* Correct the compatibility check logic

Leave a defensive versioned footer check to make sure we add new logic handling
when we add possible footer versions

Restricted VersionedFooter::from_bytes to be used inside the crate only

remove a half-baked test

* WIP.

* Return an error if index incompatible - closes #662

Enrich the error type with incompatibility

Change return type to Result<bool, TantivyError>, instead of bool

Add an Incompatibility enum that enriches the IncompatibleIndex error variant
with information, which then allows us to generate a developer-friendly hint how
to upgrade library version or switch feature flags for a different compression
algorithm

Updated changelog

Change the signature of is_compatible

Added documentation to the Incompatibility
Added a conditional test on a Footer with lz4 erroring

src/directory/mod.rs

@@ -81,6 +81,13 @@ impl<W: TerminatingWrite> TerminatingWrite for BufWriter<W> {
     }
 }
 
+#[cfg(test)]
+impl<'a> TerminatingWrite for &'a mut Vec<u8> {
+    fn terminate_ref(&mut self, _a: AntiCallToken) -> io::Result<()> {
+        self.flush()
+    }
+}
+
 /// Write object for Directory.
 ///
 /// `WritePtr` are required to implement both Write