title: Bitbucket branch source plugin fails to scan if jenkinsfile doesnt exist
body: <p>i have Bitbucket team project set up to scan the organization folder and create the pipelines, but in absence of Jenkinsfile the job fails</p>
  
  <pre><code>Checking branch master from UKCEP/amatch-acceptance
  ERROR: [Thu Jul 04 12:59:03 UTC 2019] Could not fetch sources from navigator com.cloudbees.jenkins.plugins.bitbucket.BitbucketSCMNavigator@53f18bdb
  java.io.IOException: Communication error for url: Jenkinsfile status code: 401
      at com.cloudbees.jenkins.plugins.bitbucket.server.client.BitbucketServerAPIClient.checkPathExists(BitbucketServerAPIClient.java:478)
      at com.cloudbees.jenkins.plugins.bitbucket.BitbucketSCMSource$BitbucketProbeFactory$1.exists(BitbucketSCMSource.java:1478)
      at jenkins.scm.api.SCMSourceCriteria$Probe.stat(SCMSourceCriteria.java:99)
      at org.jenkinsci.plugins.workflow.multibranch.WorkflowBranchProjectFactory$1.isHead(WorkflowBranchProjectFactory.java:74)
      at jenkins.scm.api.trait.SCMSourceRequest.process(SCMSourceRequest.java:342)
      at com.cloudbees.jenkins.plugins.bitbucket.BitbucketSCMSource.retrieveBranches(BitbucketSCMSource.java:748)
      at com.cloudbees.jenkins.plugins.bitbucket.BitbucketSCMSource.retrieve(BitbucketSCMSource.java:589)
      at jenkins.scm.api.SCMSource._retrieve(SCMSource.java:373)
      at jenkins.scm.api.SCMSource.fetch(SCMSource.java:327)
      at jenkins.branch.MultiBranchProjectFactory$BySCMSourceCriteria.recognizes(MultiBranchProjectFactory.java:263)
      at jenkins.branch.OrganizationFolder$SCMSourceObserverImpl$1.recognizes(OrganizationFolder.java:1404)
      at jenkins.branch.OrganizationFolder$SCMSourceObserverImpl$1.complete(OrganizationFolder.java:1419)
      at jenkins.scm.api.trait.SCMNavigatorRequest.process(SCMNavigatorRequest.java:254)
      at jenkins.scm.api.trait.SCMNavigatorRequest.process(SCMNavigatorRequest.java:204)
      at com.cloudbees.jenkins.plugins.bitbucket.BitbucketSCMNavigator.visitSources(BitbucketSCMNavigator.java:479)
      at jenkins.branch.OrganizationFolder.computeChildren(OrganizationFolder.java:488)
      at com.cloudbees.hudson.plugins.folder.computed.ComputedFolder.updateChildren(ComputedFolder.java:277)
      at com.cloudbees.hudson.plugins.folder.computed.FolderComputation.run(FolderComputation.java:164)
      at jenkins.branch.OrganizationFolder$OrganizationScan.run(OrganizationFolder.java:968)
      at hudson.model.ResourceController.execute(ResourceController.java:97)
      at hudson.model.Executor.run(Executor.java:429)
  [Thu Jul 04 12:59:03 UTC 2019] Finished organization scan. Scan took 11 sec
  FATAL: Failed to recompute children of EU Keying and Linking
  java.io.IOException: Communication error for url: Jenkinsfile status code: 401
      at com.cloudbees.jenkins.plugins.bitbucket.server.client.BitbucketServerAPIClient.checkPathExists(BitbucketServerAPI
  </code></pre>
  
  <p>Expected result: the repositories which don't have Jenkinsfile should be ignored 
  Actual output: the failure error mentioned above</p>
  
  <p>Jenkins version: 2.1.83
  Bitbucket branch source plugin version: 2.4.5</p>