Improve error message when --param flag is missing

Param flag (`--param`) should be treated as required when project actually uses some `{{ .param.* }}` placeholders. Otherwise it should be ignored.

#### Steps to reproduce the issue

```sh
go build cmd/build/*
cd examples/legacy-format
../../build
```

#### What's the expected result?

Error message should inform which parameter is missing and how to specify it. Example message for param named *foobar*:

```
Missing required flag: --param foobar=VALUE
```

#### What's the actual result?

```
...
Value for {{.params.param}} placeholder is not specified, available placeholders:
.service.dir, .service.name, .dirs.project, .dirs.service, .project.dir, .project.name
```