Only try to display messages from valid lines

Attempting to prevent the following exception:

    Uncaught Error: No screen line exists when converting buffer row to screen row /Applications/Atom.app/Contents/Resources/app/src/display-buffer.js:932
      module.exports.DisplayBuffer.screenPositionForBufferPosition /Applications/Atom.app/Contents/Resources/app/src/display-buffer.js:932
      module.exports.Marker.getHeadScreenPosition /Applications/Atom.app/Contents/Resources/app/src/marker.js:211
      Marker /Applications/Atom.app/Contents/Resources/app/src/marker.js:45
      module.exports.DisplayBuffer.getMarker /Applications/Atom.app/Contents/Resources/app/src/display-buffer.js:1124
      module.exports.DisplayBuffer.markBufferRange /Applications/Atom.app/Contents/Resources/app/src/display-buffer.js:1156
      module.exports.Editor.markBufferRange /Applications/Atom.app/Contents/Resources/app/src/editor.js:1186
      LinterView.display linter-view.coffee:180
      LinterView.processMessage linter-view.coffee:159
      (anonymous function) linter-view.coffee:1
      (anonymous function) linter-view.coffee:142
      Linter.processMessage linter.coffee:150
      (anonymous function) linter.coffee:127
      BufferedProcess.triggerExitCallback /Applications/Atom.app/Contents/Resources/app/src/buffered-process.js:52
      (anonymous function) /Applications/Atom.app/Contents/Resources/app/src/buffered-process.js:59
      (anonymous function) /Applications/Atom.app/Contents/Resources/app/src/buffered-process.js:105
      EventEmitter.emit events.js:129
      (anonymous function) net.js:461

Test Plan:

Can't repro that issue, but did test that error messages still work.