Improve error handling and validation for Maven resources

[gastaldi]

• Update status codes for invalid requests
• Remove test for inexistent platforms
• Update test for platforms
• Require all version numbers to be larger than 0 for validation
• Fixes #183
• Fixes https://github.com/quarkusio/quarkus/issues/32266

[src/test/java/io/quarkus/registry/app/services/MavenResourceTest.java]

• Change return status code from 400 to 404 for requests with invalid version
• Change return status code from 400 to 404 for requests with invalid snapshot version
• Change return status code from 400 to 404 for requests with invalid version in sha1 file
• Change return status code from 400 to 404 for requests with invalid snapshot version in sha1 file
• Remove test for inexistent platforms
• Update test for [src/main/java/io/quarkus/registry/app/maven/PlatformsContentProvider.java]
• Change the response status from NO_CONTENT to NOT_FOUND when no platforms are found [src/main/java/io/quarkus/registry/app/util/Version.java]
• Require major, minor, and incremental versions to be larger than 0 for validation