validate: catch problems with misspelled inputs.files and similar directives

[tiborsimko]

Current behaviour

The errors in directives such as inputs, parameters etc are not caught:

$ cd reana-demo-helloworld
$ sed -i 's,files:,fls:,g' reana.yaml 
$ git diff         
diff --git a/reana.yaml b/reana.yaml
index 50d1bd8..9597909 100644
--- a/reana.yaml
+++ b/reana.yaml
@@ -1,6 +1,6 @@
 version: 0.3.0
 inputs:
-  files:
+  fls:
     - code/helloworld.py
     - data/names.txt
   parameters:
@@ -19,5 +19,5 @@ workflow:
               --outputfile "${outputfile}"
               --sleeptime ${sleeptime}
 outputs:
-  files:
+  fls:
    - results/greetings.txt
$ reana-client validate
==> Verifying REANA specification file... /home/tibor/private/project/reana/src/reana-demo-helloworld/reana.yaml
  -> SUCCESS: Valid REANA specification file.
==> Verifying REANA specification parameters... 
  -> SUCCESS: REANA specification parameters appear valid.
==> Verifying workflow parameters and commands... 
  -> SUCCESS: Workflow parameters and commands appear valid.
==> Verifying dangerous workflow operations... 
  -> SUCCESS: Workflow operations appear valid.

This workflow appears to be valid, but it is actually not and it won't run properly.

Expected behaviour

Instead of seeing:

-> SUCCESS: Valid REANA specification file.

one would expect to see:

-> ERROR: The `reana.yaml` directive `inputs.fls` is not valid.

The validate should catch these kinds of error situations with inputs.files, inputs.parameters etc for the user and should end with an error exit status. (See also #561.)