bug#33360: Guix does not error pretty when filesystem is ro

[swedebugia]

Hi

Today I booted up my pc from standby.

Qemu was running GuixSD from a cow on an USB-disk.

It had remounted ro and I got weird errors from guix when trying to build.

All other programs just stated the obvious:

"Cannot write to bla bla - read-only filesystem."

Guix of course tried to do a lot but failed of course.

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Could we add a simple check to the commands that rely on writing to the 
filesystem?

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Cheers Swedebugia