Do not hide YAML parsing errors.

[porridge]

What this PR does / why we need it:

Before:

$ echo '"error' | ./bin/kubectl-kuttl assert /dev/stdin 
Error: file "/dev/stdin" load yaml error
$

After:

$ echo '"error' | ./bin/kubectl-kuttl assert /dev/stdin 
Error: file "/dev/stdin" load yaml error: error decoding yaml /dev/stdin: error converting YAML to JSON: yaml: line 2: found unexpected end of stream
$

Fixes #323

[kensipe]

/lgrm

agree! and thank you!