java.io.FileNotFoundException

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``` java
E/LogWriter: save:
java.io.FileNotFoundException: /data/com.github.piasy.bootstrap/performance/looper-2016-12-11_23-48-25.844.log (No such file or directory)
at java.io.FileOutputStream.open(Native Method)
at java.io.FileOutputStream.(FileOutputStream.java:221)
at java.io.FileOutputStream.(FileOutputStream.java:140)
at com.github.moduth.blockcanary.LogWriter.save(LogWriter.java:108)
at com.github.moduth.blockcanary.LogWriter.save(LogWriter.java:56)
at com.github.moduth.blockcanary.BlockCanaryInternals$1.onBlockEvent(BlockCanaryInternals.java:63)
at com.github.moduth.blockcanary.LooperMonitor$1.run(LooperMonitor.java:74)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.os.HandlerThread.run(HandlerThread.java:61)
```

Start by reviewing LogWriter.java at lines 56 and 108, then trace the save call from BlockCanaryInternals$1.onBlockEvent at line 63. Reproduce the FileNotFoundException with the reported log path and determine the expected behavior when the path is unavailable. Done means the failure is addressed and the relevant save flow is covered by an appropriate test.

Written by the indexing model from the issue text.