add test for error handling

.golangci.yml

@@ -13,6 +13,7 @@ linters:
 
   - wrapcheck
   - ireturn
+  - forbidigo
 
   # linters deprecated
   - deadcode

sqlite_zstd_suite_test.go

@@ -103,4 +103,15 @@ var _ = Describe("SqliteZSTD", func() {
 
 		waiter.Wait()
 	})
+
+	When("file does not exist", func() {
+		It("returns an error", func() {
+			client, err := sql.Open("sqlite3", "file:some.db?vfs=zstd")
+			Expect(err).ToNot(HaveOccurred())
+			defer client.Close()
+
+			row := client.QueryRow("SELECT * FROM entries ORDER BY RANDOM() LIMIT 1;")
+			Expect(row.Err()).To(HaveOccurred())
+		})
+	})
 })

vfs.go

@@ -34,17 +34,17 @@ func (z *ZstdVFS) FullPathname(name string) string {
 func (z *ZstdVFS) Open(name string, flags sqlite3vfs.OpenFlag) (sqlite3vfs.File, sqlite3vfs.OpenFlag, error) {
 	file, err := os.Open(name)
 	if err != nil {
-		return nil, 0, fmt.Errorf("could not open file: %w", err)
+		return nil, 0, sqlite3vfs.CantOpenError
 	}
 
 	decoder, err := zstd.NewReader(nil)
 	if err != nil {
-		return nil, 0, fmt.Errorf("could not create reader: %w", err)
+		return nil, 0, sqlite3vfs.CantOpenError
 	}
 
 	seekable, err := seekable.NewReader(file, decoder)
 	if err != nil {
-		return nil, 0, fmt.Errorf("could not create seekable: %w", err)
+		return nil, 0, sqlite3vfs.CantOpenError
 	}
 
 	return &ZstdFile{