Rename package srverrors to httperror

- Package names are preferred to be in singular form¹.
- Since errors are just for HTTP and not generic, I changed the base name to
'http'.
- I didn't name it simply 'error', because it would then conflict with the
- standard 'error' interface.

¹: https://rakyll.org/style-packages/

internal/server/httperror/error.go

@@ -0,0 +1,24 @@
+package httperror
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+// A ServerErrMessage represents the reason why the error happened
+type ServerErrMessage struct {
+	Reason string `json:"reason"`
+}
+
+// ErrorJSON writes the provided error as a JSON body to the provided
+// http.ResponseWriter, after setting the appropriate Content-Type header
+func ErrorJSON(w http.ResponseWriter, error string, code int) {
+	body, _ := json.Marshal(
+		ServerErrMessage{
+			Reason: error,
+		},
+	)
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.WriteHeader(code)
+	_, _ = w.Write(body)
+}