give better errors for broken templates

[kadoban]

fixes #1535

[kadoban]

Upon review, the error messages sound rather badly worded; I'll fix them up in the morning.

[kadoban]

I think I'm happier with these errors, and it seems to work fine:

~/testing$ touch blah.hsfiles
~/testing$ stack new blah blah                 
Loading local template "blah" to create project "blah" in blah/ ...
The template "blah" is invalid and could not be used. The error was: "Template does not contain any files"
~/testing$ printf "{-# START_FILE BOGUS #-}\n" > blah.hsfiles
~/testing$ stack new blah blah
Loading local template "blah" to create project "blah" in blah/ ...
The template "blah" is invalid and could not be used. The error was: "Template does not contain a .cabal file"
~/testing$ printf "a b c d e" > blah.hsfiles
~/testing$ stack new blah blah
Loading local template "blah" to create project "blah" in blah/ ...
The template "blah" is invalid and could not be used. The error was: "InvalidInput "a b c d e""

Note that although the empty template error is technically redundant (it'll obviously be missing a .cabal file too in that case), I think it's probably still useful since it probably points to an empty template rather than one that someone forgot to add a .cabal file to.

[mgsloan]

LGTM, thanks!