improve backtrace rendering #12574

Job	Run time
coverage report	6m 51s
bootstrap build	3m 1s
style checks	5m 7s
test (i686-pc-windows-msvc)	24m 11s
test (aarch64-apple-darwin)	26m 14s
test (x86_64-unknown-linux-gnu)	23m 29s
test (aarch64-unknown-linux-gnu)	19m 24s
test (aarch64-pc-windows-msvc)	20m 5s
test (armv7-unknown-linux-gnueabihf)	17m 6s
test (i686-unknown-linux-gnu)	19m 31s
automatic pull from rustc	0s
conclusion	3s
cronjob failure notification	-1s
	2h 45m 1s

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