Handle db deletion in couch_db:load_validation_funs

Previously there were few problems with load_validation_funs
in the case when clustered database is deleted.

- the calls to load_validation_funs were failing with `internal_server` error [1]
- the deleted database stayed opened because:
  - the caller of the load_validation_funs (update_doc) stayed alive
  - the main_pid of the deleted database wasn't killed either
- there was an infinite loop in ddoc_cache_entry trying to recover ddoc from deleted database

The solution is:
- do not call `recover` for deleted database
- close `main_pid`
- use `erlang:error` to crash the caller

[1] - The stack trace was:
```
{database_does_not_exist,[
    {mem3_shards,load_shards_from_db,"bailey/meta",[
        {file,"src/mem3_shards.erl"},{line,394}]},
    {mem3_shards,load_shards_from_disk,1,[
        {file,"src/mem3_shards.erl"},{line,369}]},
    {mem3_shards,for_db,2,[
        {file,"src/mem3_shards.erl"},{line,54}]},
    {fabric_view_all_docs,go,5,[
        {file,"src/fabric_view_all_docs.erl"},{line,24}]},
    {ddoc_cache_entry_validation_funs,recover,1,[
        {file,"src/ddoc_cache_entry_validation_funs.erl"},{line,33}]},
    {ddoc_cache_entry,do_open,1,[
        {file,"src/ddoc_cache_entry.erl"},{line,294}]}]}
```

src/couch/src/couch_db.erl

@@ -864,6 +864,9 @@ load_validation_funs(#db{main_pid=Pid, name = <<"shards/", _/binary>>}=Db) ->
         {'DOWN', Ref, _, _, {ok, Funs}} ->
             gen_server:cast(Pid, {load_validation_funs, Funs}),
             Funs;
+        {'DOWN', Ref, _, _, {database_does_not_exist, _StackTrace}} ->
+            ok = couch_server:close_db_if_idle(Db#db.name),
+            erlang:error(database_does_not_exist);
         {'DOWN', Ref, _, _, Reason} ->
             couch_log:error("could not load validation funs ~p", [Reason]),
             throw(internal_server_error)

src/ddoc_cache/src/ddoc_cache_entry.erl

@@ -106,11 +106,14 @@ open(Pid, Key) ->
             {open_error, {T, R, S}} ->
                 erlang:raise(T, R, S)
         end
-    catch exit:_ ->
-        % Its possible that this process was evicted just
-        % before we tried talking to it. Just fallback
-        % to a standard recovery
-        recover(Key)
+    catch
+        error:database_does_not_exist ->
+            erlang:error(database_does_not_exist);
+        exit:_ ->
+            % Its possible that this process was evicted just
+            % before we tried talking to it. Just fallback
+            % to a standard recovery
+            recover(Key)
     end.