Throw checked exception when password authentication fails

[zbynek]

Changes exception from unchecked to checked when basic auth access it attempted.

At the moment, accessing the server with (invalid) basic authentication results in long stacktrace in the logs:

org.gitlab4j.api.GitLabApiException: 401 Unauthorized
	at PluginClassLoader for gitlab-api//org.gitlab4j.api.AbstractApi.validate(AbstractApi.java:784)
	at PluginClassLoader for gitlab-api//org.gitlab4j.api.AbstractApi.get(AbstractApi.java:262)
	at PluginClassLoader for gitlab-api//org.gitlab4j.api.UserApi.getCurrentUser(UserApi.java:700)
	at PluginClassLoader for gitlab-oauth//org.jenkinsci.plugins.GitLabAuthenticationToken.<init>(GitLabAuthenticationToken.java:101)
	at PluginClassLoader for gitlab-oauth//org.jenkinsci.plugins.GitLabSecurityRealm$1.authenticate(GitLabSecurityRealm.java:478)
Caused: java.lang.RuntimeException
	at PluginClassLoader for gitlab-oauth//org.jenkinsci.plugins.GitLabSecurityRealm$1.authenticate(GitLabSecurityRealm.java:482)
	at jenkins.security.BasicHeaderRealPasswordAuthenticator.authenticate2(BasicHeaderRealPasswordAuthenticator.java:60)
	at jenkins.security.BasicHeaderProcessor.doFilter(BasicHeaderProcessor.java:92)
	at hudson.security.ChainedServletFilter2$1.doFilter(ChainedServletFilter2.java:99)
	at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:117)
	at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:87)
	at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:63)
	at hudson.security.ChainedServletFilter2$1.doFilter(ChainedServletFilter2.java:99)
	at hudson.security.ChainedServletFilter2.doFilter(ChainedServletFilter2.java:111)
	at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:173)

Avoiding the use of unchecked exception fixes it.

Testing done

TBD

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• [n/a] Link to relevant issues in GitHub or Jira
• [n/a] Link to relevant pull requests, esp. upstream and downstream changes
• [n/a] Ensure you have provided tests that demonstrate the feature works or the issue is fixed