Add check if migrations dir exists before copying

[infomiho]

Closes #1352

[Martinsos]

Nice @infomiho ! I approved, but also left one/two comments that possibly improve code a bit, take a look and do as you like with them.

[Martinsos]

Suggested change

	copyMigrationsBackToSource genProjectRootDirAbs dbMigrationsDirInWaspProjectDirAbs = do
	doesMigrationsDirExist <- doesDirectoryExist $ SP.fromAbsDir genProjectMigrationsDir
	if doesMigrationsDirExist
	then copyMigrationsDir
	else return $ Right ()
	where
	copyMigrationsDir =
	copyDirectoryRecursive genProjectMigrationsDir waspMigrationsDir >> return (Right ())
	`catch` (\e -> return $ Left $ show (e :: P.PathException))
	`catch` (\e -> return $ Left $ show (e :: IOError))
	copyMigrationsBackToSource genProjectRootDirAbs dbMigrationsDirInWaspProjectDirAbs = do
	doesDirectoryExist (SP.fromAbsDir genProjectMigrationsDir) >>= \case ->
	False -> return $ Right ()
	True -> copyDirectoryRecursive genProjectMigrationsDir waspMigrationsDir >> return (Right ())
	`catch` (\e -> return $ Left $ show (e :: P.PathException))
	`catch` (\e -> return $ Left $ show (e :: IOError))

[Martinsos]

We could also consider renaming whole functoin to copyMigrationsBackToSourceIfTheyExist, but I am not sure, we can also skip it maybe.