xharness throws an exception when there is no install log, instead we should log an error and continue

xharness does not contain handling for the scenario where there is no install log. Instead, an exception is thrown. This obscures the actual cause of failures.

Commits have run into this issues on device tests only, Jenkins jobs have succeeded for some commits running into this failure.

https://github.com/xamarin/xamarin-macios/commit/61580b15af1d9944f3b8bd1ca014b0428f11cf8d

```
Failure:
Harness exception for 'framework-test': System.IO.FileNotFoundException: Could not find file "/Users/xamarinqa/myagent/_work/1/s/jenkins-results/tests/framework-test/824/install-20200506_235011.log"
File name: '/Users/xamarinqa/myagent/_work/1/s/jenkins-results/tests/framework-test/824/install-20200506_235011.log'
at System.IO.FileStream..ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share, System.Int32 bufferSize, System.Boolean anonymous, System.IO.FileOptions options) [0x00259] in /Users/builder/jenkins/workspace/build-package-osx-mono/2020-02/external/bockbuild/builds/mono-x64/mcs/class/corlib/System.IO/FileStream.cs:274
at System.IO.FileStream..ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share) [0x00000] in /Users/builder/jenkins/workspace/build-package-osx-mono/2020-02/external/bockbuild/builds/mono-x64/mcs/class/corlib/System.IO/FileStream.cs:91
at (wrapper remoting-invoke-with-check) System.IO.FileStream..ctor(string,System.IO.FileMode,System.IO.FileAccess,System.IO.FileShare)
at Microsoft.DotNet.XHarness.iOS.Shared.Logging.LogFile.GetReader () [0x00001] in /Users/xamarinqa/myagent/_work/1/s/tests/xharness/Microsoft.DotNet.XHarness.iOS.Shared/Logging/LogFile.cs:58
at Microsoft.DotNet.XHarness.iOS.Shared.Logging.AppInstallMonitorLog.GetReader () [0x00000] in /Users/xamarinqa/myagent/_work/1/s/tests/xharness/Microsoft.DotNet.XHarness.iOS.Shared/Logging/AppInstallMonitorLog.cs:48
at Xharness.TestTasks.RunDevice.RunTestAsync () [0x0073c] in /Users/xamarinqa/myagent/_work/1/s/tests/xharness/TestTasks/RunDevice.cs:133
at Xharness.TestTasks.RunDevice.RunTestAsync () [0x010e0] in /Users/xamarinqa/myagent/_work/1/s/tests/xharness/TestTasks/RunDevice.cs:219
at Xharness.TestTasks.RunTest.ExecuteAsync () [0x001b9] in /Users/xamarinqa/myagent/_work/1/s/tests/xharness/TestTasks/RunTest.cs:107
at Microsoft.DotNet.XHarness.iOS.Shared.Tasks.TestTasks.RunInternalAsync () [0x00226] in /Users/xamarinqa/myagent/_work/1/s/tests/xharness/Microsoft.DotNet.XHarness.iOS.Shared/Tasks/TestTask.cs:282
```