Improve no readme error message

[mxsdev]

Solves #108

Error is now formatted like:

readme file does not exist: ./cmd/README.md

[adambabik]

Also, we can add an integration test:

--- a/testdata/script/basic.txtar
+++ b/testdata/script/basic.txtar
@@ -6,6 +6,10 @@ exec runme ls --allow-unknown
 cmp stdout golden-list-allow-unknown.txt
 ! stderr .
 
+! exec runme ls --filename nonexistent.md
+stderr 'failed to open markdown file .*/nonexistent.md: no such file or directory'
+! stdout .

You can execute it like this:

go test -timeout 10s -run ^TestRunme$ .

[mxsdev]

@adambabik Done!