Improve error message when the Git repo zip resource can not be found

Messages like this NPE are not very clear:

[info] ! handle ASCII in ISO-8859-1
[error]    NullPointerException:   (package.scala:37)
[error] com.madgag.git.test.package$.unpackRepoAndGetGitDir(package.scala:37)
[error] com.madgag.git.test.package$.unpackRepo(package.scala:31)
[error] com.madgag.git.bfg.cleaner.RepoRewriteSpec$$anonfun$5.com$madgag$git$bfg$cleaner$RepoRewriteSpec$$anonfun$$textReplacementOf$1(RepoRewriteSpec.scala:129)
[error] com.madgag.git.bfg.cleaner.RepoRewriteSpec$$anonfun$5$$anonfun$apply$28.apply(RepoRewriteSpec.scala:152)
[error] com.madgag.git.bfg.cleaner.RepoRewriteSpec$$anonfun$5$$anonfun$apply$28.apply(RepoRewriteSpec.scala:152)
[info]
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