cmd/go: misleading error message when build cache runs out of disk space

[millerresearch]

Running make.bash on linux/amd64 using current go source produced weird error messages:

$ GOOS=plan9 GOARCH=arm GOARM=7 ./make.bash
Building Go cmd/dist using /tmp/go. (go1.19rc2 linux/amd64)
Building Go toolchain1 using /tmp/go.
Building Go bootstrap cmd/go (go_bootstrap) using Go toolchain1.
Building Go toolchain2 using go_bootstrap and Go toolchain1.
Building Go toolchain3 using go_bootstrap and Go toolchain2.
Building commands for host, linux/amd64.
/usr/local/go/src/runtime/cgocall.go:90:2: package runtime/internal/sys is not in GOROOT (/usr/local/go/src/runtime/internal/sys)
/usr/local/go/src/cmd/api/api.go:15:2: package go/build is not in GOROOT (/usr/local/go/src/go/build)
/usr/local/go/src/internal/testenv/testenv.go:19:2: package internal/platform is not in GOROOT (/usr/local/go/src/internal/platform)
/usr/local/go/src/testing/benchmark.go:11:2: package internal/sysinfo is not in GOROOT (/usr/local/go/src/internal/sysinfo)
package cmd/cgo is not in GOROOT (/usr/local/go/src/cmd/cgo)
package cmd/dist is not in GOROOT (/usr/local/go/src/cmd/dist)
/usr/local/go/src/cmd/go/internal/base/base.go:19:2: package cmd/go/internal/cfg is not in GOROOT (/usr/local/go/src/cmd/go/internal/cfg)
go tool dist: FAILED: /usr/local/go/pkg/tool/linux_amd64/go_bootstrap install cmd: exit status 1

Eventually I realised the disk partition containing ~/.cache/go-build was full, and after go clean -cache I was able to re-run the build successfully.

It appears something in the build cache maintenance code might not be reporting write errors. The error being reported is very wide of the mark.