Ensure provided Git repository and path to Dockerfile are valid

[davidkopp]

Currently if the import of a MICO service with the GitHubCrawler is used, the URL to the repository and the provided path to the Dockerfile is validated.
Validation of the path to the Dockerfile:

mico/mico-core/src/main/java/io/github/ust/mico/core/service/GitHubCrawler.java

Line 115 in ec110a1

if (existsFileInGithubRepo(gitHubRepositoryApiUrl, dockerfilePath)) {

However this is not the case if a MICO service is updated manually.