Crashes when version string is not in proper semver format

[joscha-alisch]

For a buildpack that had a typo in the version string ("v.9.1.6" instead of "v9.1.6"), the update action crashed with the following exception:

04:34:25.121 INFO  c.s.n.BuildpackVersionChecker - Performing checks
Exception in thread "main" java.lang.IllegalArgumentException: Invalid version string [.9.1.6]
	at net.swiftzer.semver.SemVer$Companion.parse(SemVer.kt:28)
	at com.springernature.newversion.SemanticVersion.toSemVer(Manifest.kt:46)
	at com.springernature.newversion.BuildpackUpdate.hasUpdate(BuildpackUpdate.kt:11)
	at com.springernature.newversion.BuildpackVersionChecker.performChecks(BuildpackVersionChecker.kt:41)
	at com.springernature.newversion.MainKt.main(Main.kt:17)
	at com.springernature.newversion.MainKt.main(Main.kt)

The trigger was an invalid tag name for the latest release on GitHub, not a typo in the manifest. In this case the buildpack had a git tag v.9.1.6.

Instead of crashing, it should probably do one of those things:

• Act as if there is no update at all
• Print a warning, that something appears to be wrong - but don't fail
• Fail with a proper error message stating that the user should alert the buildpack maintainers