Flank should reflect gcloud exit codes

[adamfilipow92]

Fixes #802

Test Plan

How do we know the code works?

Flank exit codes reflect gcloud exit codes

Comparision

Documentation about the current state can be found here, please keep in mind this is only my interpretation and feel free to request changes.

Exit code	Gcloud	Flank
0	All test executions passed.	All tests passed
1	A general failure occurred. Possible causes include: a filename that does not exist or an HTTP/network error.	All matrices finished but at least one test failed or inconclusive.
2	Testing exited because unknown commands or arguments were provided.	Usually indicates missing or wrong usage of flags, incorrect parameters, errors in config files.
3		At least one matrix not finished (usually a FTL internal error) or unexpected error occurred.
10	One or more test cases (tested classes or class methods) within a test execution did not pass.
15	Firebase Test Lab could not determine if the test matrix passed or failed, because of an unexpected error.
18	The test environment for this test execution is not supported because of incompatible test dimensions. This error might occur if the selected Android API level is not supported by the selected device type.
19	The test matrix was canceled by the user.
20	A test infrastructure error occurred.

Mapping plan Flank -> Gcloud and test plan

Status	Gcloud Code	Old Flank Code	Description
✅	0	0	All test executions passed.
✅	10	1	One or more test cases (tested classes or class methods) within a test execution did not pass.
✅	1	2	Apk, yml not found or an HTTP/network error
✅	2	2	Unknown commands
✅	18	2	The test environment for this test execution is not supported because of incompatible test dimensions. Eg. device not support selected API level
✅	15	3	Unexpected errors from gcloud
✅	19	3	The test matrix was canceled by the user.
✅	20	3	A test infrastructure error occurred.

Checklist

• Exit codes changed
• Check exceptions what Flank throws and refractor to types reflected gcloud exit codes
• Documented
• Unit tested
• release_notes.md updated
• readme.md updated