Exit with an error if $CABAL_SANDBOX_CONFIG does not point to extant file

[BardurArantsson]

I think this fixes #2224. I haven't done extensive manual testing, but the tests all (obviously) still pass.

Example output:

cabal: Cabal sandbox file in $CABAL_SANDBOX_CONFIG does not exist: foo

the "foo" is the content of the env variable.

I should note that the error occurs quite "aggressively", i.e. when the settings are read, and not when they are actually used -- so even things like "cabal info" will fail if the env variable is invalid. I think a more relaxed would be a lot more error-prone since it would be easy to miss cases.

[BardurArantsson]

Hm. I see there are some actual possibly-real failures here. I'll investigate tomorrow.

[BardurArantsson]

Ah, yes, FTP.

Alright, I think this should be good to go.

[edsko]

I've submitted a PR that undoes this commit, because it makes cabal completely unusable when CABAL_SANDBOX_CONFIG is set. This check happens much too early, so that for instance we get

# ../dist/build/cabal/cabal sandbox init
cabal: Cabal sandbox file in $CABAL_SANDBOX_CONFIG does not exist:
./cabal.sandbox.config-7.8.4

Moreover, even if we initialize the sandbox using an older cabal

# ~/opt/cabal/cabal-1.22.0.0 sandbox init
Writing a default package environment file to ./cabal.sandbox.config-7.8.4
Creating a new sandbox at
/Users/e/wt/projects/tuf/cabal/cabal-install/t/.cabal-sandbox

trying to install any packages into the sandbox fails:

# ../dist/build/cabal/cabal install mtl
Resolving dependencies...
Notice: installing into a sandbox located at
/Users/e/wt/projects/tuf/cabal/cabal-install/t/.cabal-sandbox
Configuring transformers-0.4.3.0...
Failed to install transformers-0.4.3.0
Build log ( /Users/e/wt/projects/tuf/cabal/cabal-install/t/.cabal-sandbox/logs/transformers-0.4.3.0.log ):
cabal: Cabal sandbox file in $CABAL_SANDBOX_CONFIG does not exist:
./cabal.sandbox.config-7.8.4
cabal: Error: some packages failed to install:
mtl-2.2.1 depends on transformers-0.4.3.0 which failed to install.
transformers-0.4.3.0 failed during the configure step. The exception was:
ExitFailure 1

[BardurArantsson]

Absolutely fair enough to just revert. I guess my testing wasn't sufficient :(. I guess this is really a cue that we really need to expand the test suite and to make it easier to add tests.

(I don't think I'll get around to implementing this properly any time soon.)