500 error is returned when mandatory argument is not provided

[Alex Sch]

Describe the bug
Proper error handling is missing

To Reproduce
running the example (functions):

$ go run main.go daemon -f ../../examples/functions/config.hcl 

making the request (missing "name" parameter):

[POST] http//localhost:8080/numeric

{
    "age": 35
}

getting status 500 Internal Server Error and empty response body

Application log:

1622023136544832264	info	listener.http	serving HTTP listener	{"addr": ":8080"}
1622023161881029516	error	flow.numeric	an error occurred, executing rollback	{"error": "value is not set"}
github.com/jexia/semaphore/v2/pkg/broker/logger.Error
	/home/alma/Projects/semaphore/pkg/broker/logger/logger.go:85
github.com/jexia/semaphore/v2/pkg/flow.(*Manager).Do
	/home/alma/Projects/semaphore/pkg/flow/flow.go:154
github.com/jexia/semaphore/v2/pkg/transport/http.(*Handle).HTTPFunc
	/home/alma/Projects/semaphore/pkg/transport/http/listener.go:252
github.com/julienschmidt/httprouter.(*Router).ServeHTTP
	/home/alma/Projects/semaphore/vendor/github.com/julienschmidt/httprouter/router.go:387
github.com/jexia/semaphore/v2/pkg/transport/http.(*Listener).Serve.func1
	/home/alma/Projects/semaphore/pkg/transport/http/listener.go:68
net/http.HandlerFunc.ServeHTTP
	/usr/lib/go/src/net/http/server.go:2069
net/http.serverHandler.ServeHTTP
	/usr/lib/go/src/net/http/server.go:2887
net/http.(*conn).serve
	/usr/lib/go/src/net/http/server.go:1952

Expected
status 400 Bad Request and response body containing:

{
    "error": "value is not set"
}