factor out remaining occurrences of ExitFail not in cmd/main.go or paths used by just CLI

[naggie]

And possibly Must* style methods.

This is necessary due to the ongoing development of a dstask web interface by @botto

[dontlaugh]

Here is a ripgrep command that might find what we are looking for in this issue, e.g. "ExitFail" not under the path cmd/

 rg -g '!cmd' -F ExitFail

Results:

task.go
279:			ExitFail("Could not remove task %s: %v", filepath, err)
292:			ExitFail("Failed to marshal task %s", t)
297:			ExitFail("Failed to write task %s", t)
312:				ExitFail("Could not remove task %s: %v", filepath, err)

localstate.go
64:		ExitFail("Failed to open %s for writing: ", filePath)
73:		ExitFail("Failed to encode state gob: %s, %s", filePath, err)
81:		ExitFail("Failed to open %s for reading: ", filePath)
90:		ExitFail("Failed to parse state gob: %s, %s", filePath, err)

git.go
21:		ExitFail("Failed to run git cmd.")
34:		ExitFail("error: %s", err)
80:			ExitFail("Failed to create directory in git repository")
91:		ExitFail("git required, please install")
102:			ExitFail("Failed to create directory in git repository")

query.go
188:			ExitFail("Could not apply q2, project conflict")
196:			ExitFail("Could not apply q2, priority conflict")

commands.go
301:				ExitFail("Could not write to stdout: %v", err)

taskset.go
148:		ExitFail("%s, task %s", err, task.UUID)
204:		ExitFail(err.Error())
274:		ExitFail(err.Error())

util.go
16:func ExitFail(format string, a ...interface{}) {
34:	ExitFail("Aborted.")
92:		ExitFail("Could not create temporary file to edit")
100:		ExitFail("Could not write to temporary file to edit")
105:		ExitFail("Failed to run $EDITOR")
111:		ExitFail("Could not read back temporary edited file")
175:		ExitFail("unsupported platform")
179:		ExitFail("Failed to open browser")
204:		ExitFail("Not a TTY")

display.go
70:		ExitFail("No matching tasks in given context or filter.")