Improve error message when --param flag is missing

[qzb]

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

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