cleanups: Eliminate `errstr` and (nearly) eliminate `Unexpected` #2117

Job	Run time
Docs - stable toolchain (recent)	36s
Docs - nightly toolchain (recent)	38s
Bench - nightly toolchain (recent)	3m 11s
Embedded	1m 28s
Format - nightly toolchain	12s
Integration tests - stable toolchain (26_0)	48s
Integration tests - stable toolchain (25_2)	52s
Test - 1.63.0 toolchain (minimal)	26m 39s
Integration tests - stable toolchain (25_1)	52s
Lint - nightly toolchain (recent)	47s
Test - 1.63.0 toolchain (recent)	27m 9s
Integration tests - stable toolchain (25_0)	50s
Integration tests - stable toolchain (24_2)	1m 5s
Integration tests - stable toolchain (24_1)	52s
Test - nightly toolchain (minimal)	15m 5s
Test - stable toolchain (minimal)	16m 47s
Integration tests - stable toolchain (24_0_1)	47s
Test - nightly toolchain (recent)	15m 29s
Test - stable toolchain (recent)	16m 49s
Integration tests - stable toolchain (23_2)	51s
Integration tests - stable toolchain (23_1)	50s
Integration tests - stable toolchain (23_0)	46s
Integration tests - stable toolchain (22_1)	47s
Integration tests - stable toolchain (22_0)	47s
Integration tests - stable toolchain (0_21_2)	52s
Integration tests - stable toolchain (0_20_2)	49s
Integration tests - stable toolchain (0_19_1)	50s
Integration tests - stable toolchain (0_18_1)	45s
Integration tests - stable toolchain (0_17_1)	48s
	2h 19m 1s

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