Fix #1816 - give more descriptive errors when .cabal file cannot be found #1824
This will give better errors when a .cabal file cannot be found when:
The first patch is a simple tidy-up: removing some inline, repeated strings, and manual layout formatting.
Avoid inline strings and manual spacing - use already-declared key variables and unwords
Particularly when dealing with add-source dependencies, it is useful to know in which directory cabal can't find a .cabal file, see: #1816 Errors are also improved when installing local packages, e.g. `cabal install another/directory`.
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