update docs and wrap pebble.ExecError with custom error #2422

Job	Run time
integration-tests / Plan	6s
integration-tests / Build charm (synapse)	11m 6s
integration-tests / Build rock (synapse)	21m 44s
integration-tests / Plan Image Scanning	3s
integration-tests / Integration tests / Integration tests (test_charm)	19m 23s
integration-tests / Integration tests / Integration tests (test_nginx)	12m 13s
integration-tests / Integration tests / Integration tests (test_s3)	24m 50s
integration-tests / Integration tests / Integration tests (test_scaling)	16m 28s
integration-tests / Integration tests / Integration tests (test_matrix_auth)	13m 6s
integration-tests / Scan Image (ghcr.io-canonical-synapse-4142a4055d88c05ea60194be0476ca8cfcd98d90-_3.0_amd64.tar)	1m 16s
integration-tests / Required Integration Test Status Checks	1s
allure-report	0s
	2h 0m 16s

Provide feedback