feat: Fail schema cache lookup with invalid db-schemas config

Previously, we'd silently report "200 OK" on the root endpoint, but
would never return any endpoints from the schema cache.

Now the schema cache query fails because of the ::regnamespace cast.

src/PostgREST/Query/SqlFragment.hs

@@ -157,10 +157,10 @@ pgBuildArrayLiteral vals =
 
 -- TODO: refactor by following https://github.com/PostgREST/postgrest/pull/1631#issuecomment-711070833
 pgFmtIdent :: Text -> SQL.Snippet
-pgFmtIdent x = SQL.sql $ escapeIdent x
+pgFmtIdent x = SQL.sql . encodeUtf8 $ escapeIdent x
 
-escapeIdent :: Text -> ByteString
-escapeIdent x = encodeUtf8 $ "\"" <> T.replace "\"" "\"\"" (trimNullChars x) <> "\""
+escapeIdent :: Text -> Text
+escapeIdent x = "\"" <> T.replace "\"" "\"\"" (trimNullChars x) <> "\""
 
 -- Only use it if the input comes from the database itself, like on `jsonb_build_object('column_from_a_table', val)..`
 pgFmtLit :: Text -> Text
@@ -181,7 +181,7 @@ trimNullChars = T.takeWhile (/= '\x0')
 -- >>> escapeIdentList ["schema_1", "schema_2", "SPECIAL \"@/\\#~_-"]
 -- "\"schema_1\", \"schema_2\", \"SPECIAL \"\"@/\\#~_-\""
 escapeIdentList :: [Text] -> ByteString
-escapeIdentList schemas = BS.intercalate ", " $ escapeIdent <$> schemas
+escapeIdentList schemas = BS.intercalate ", " $ encodeUtf8 . escapeIdent <$> schemas
 
 asCsvF :: SQL.Snippet
 asCsvF = asCsvHeaderF <> " || '\n' || " <> asCsvBodyF