Print out better error message if apk file or directory doesn't exist

[runningcode]

No description provided.

[kenyee]

Another thing that would help is printing the path to the report file when flank has been run.
Here's an example what it prints now:

  Smart Flank cache hit: 0% (0 / 5)
  Shard times: 50s

  5 tests / 1 shard

  1 matrix ids created in 0m 5s
  https://console.developers.google.com/storage/browser/test-lab-2f77bw9wh508q-im9i6j421bn3i/2019-04-04_16-46-59.791000_Ytht/

PollMatrices
  0m  0s matrix-3if2rb73y0h97 VALIDATING
  0m 15s matrix-3if2rb73y0h97 PENDING
  1m  0s matrix-3if2rb73y0h97 Starting attempt 1.
  1m  0s matrix-3if2rb73y0h97 RUNNING
  2m  1s matrix-3if2rb73y0h97 Started logcat recording.
  2m  1s matrix-3if2rb73y0h97 Preparing device.
  2m 16s matrix-3if2rb73y0h97 Installing apps.
  2m 31s matrix-3if2rb73y0h97 Retrieving Pre-Test Package Stats information from the device.
  2m 31s matrix-3if2rb73y0h97 Retrieving Performance Environment information from the device.
  2m 31s matrix-3if2rb73y0h97 Started crash detection.
  2m 31s matrix-3if2rb73y0h97 Started crash monitoring.
  2m 31s matrix-3if2rb73y0h97 Started performance monitoring.
  2m 31s matrix-3if2rb73y0h97 Started video recording.
  2m 31s matrix-3if2rb73y0h97 Starting instrumentation test.
  3m 32s matrix-3if2rb73y0h97 Completed instrumentation test.
  3m 47s matrix-3if2rb73y0h97 Stopped performance monitoring.
  4m  2s matrix-3if2rb73y0h97 Stopped crash monitoring.
  4m  2s matrix-3if2rb73y0h97 Stopped logcat recording.
  4m  2s matrix-3if2rb73y0h97 Retrieving Post-test Package Stats information from the device.
  4m  2s matrix-3if2rb73y0h97 Done. Test time = 51 (secs)
  4m  2s matrix-3if2rb73y0h97 Starting results processing. Attempt: 1
  4m 17s matrix-3if2rb73y0h97 FINISHED

FetchArtifacts
  .
  Updating matrix file

CostReport
  Virtual devices
    $0.02 for 1m

MatrixResultsReport
  0 / 1 (0.00%)
  1 matrices failed

  matrix-3if2rb73y0h97 
  https://console.firebase.google.com/project/android-kayak-app/testlab/histories/bh.f4df70e21d196d2c/matrices/5556466349258360347/executions/bs.22114649292bf9c9

AGP's lint prints out the filename of the report after it has been run.

Looks like it's dumping the report into KayakTravelApp/build/fladle/results/2019-04-04_16-46-59.791000_Ytht/HtmlErrorReport.html