Unable to uninstall error message prints strangely in backfill.py

[mcomella]

I didn't have fenix installed so I rightly saw an error that the uninstall failed. However, it's formatted strangey (e.g. newlines aren't processed, is printed as a python string literal rather than directly to the console):

Something with the uninstalling org.mozilla.fenix went wrong. The associated error message was:

b'Exception occurred while executing:\njava.lang.IllegalArgumentException: Unknown package: org.mozilla.fenix\n\tat com.android.server.pm.Settings.isOrphaned(Settings.java:4527)\n\tat com.android.server.pm.PackageManagerService.isOrphaned(PackageManagerService.java:23001)\n\tat com.android.server.pm.PackageManagerService.deletePackageVersioned(PackageManagerService.java:19870)\n\tat com.android.server.pm.PackageInstallerService.uninstall(PackageInstallerService.java:745)\n\tat com.android.server.pm.PackageManagerShellCommand.runUninstall(PackageManagerShellCommand.java:937)\n\tat com.android.server.pm.PackageManagerShellCommand.onCommand(PackageManagerShellCommand.java:134)\n\tat android.os.ShellCommand.exec(ShellCommand.java:96)\n\tat com.android.server.pm.PackageManagerService.onShellCommand(PackageManagerService.java:23320)\n\tat android.os.Binder.shellCommand(Binder.java:594)\n\tat android.os.Binder.onTransact(Binder.java:492)\n\tat android.content.pm.IPackageManager$Stub.onTransact(IPackageManager.java:2707)\n\tat com.android.server.pm.PackageManagerService.onTransact(PackageManagerService.java:3955)\n\tat android.os.Binder.execTransact(Binder.java:697)'

[mcomella]

Addressed with https://github.com/mozilla-mobile/perf-tools/pull/15/