refactor: Move HandleError to dbwrapper implementation

Make it a static function in dbwrapper.cpp, since it is not used
elsewhere and when left in the header, would expose a leveldb type.

The context of this commit is an effort to decouple the dbwrapper header
file from leveldb includes. To this end, the includes are moved to the
dbwrapper implementation file. This is done as part of the kernel
project to reduce the number of required includes for users of the
kernel.

src/dbwrapper.h

@@ -24,7 +24,6 @@
 #include <vector>
 namespace leveldb {
 class Env;
-class Status;
 }
 
 static const size_t DBWRAPPER_PREALLOC_KEY_SIZE = 64;
@@ -67,10 +66,6 @@ class CDBWrapper;
  */
 namespace dbwrapper_private {
 
-/** Handle database error by throwing dbwrapper_error exception.
- */
-void HandleError(const leveldb::Status& status);
-
 /** Work around circular dependency, as well as for testing in dbwrapper_tests.
  * Database obfuscation should be considered an implementation detail of the
  * specific database.