[PATCH] Detect malformed yaml sequence in yaml::Input::beginSequence()

[William Fisher]

When reading a yaml::SequenceTraits object, YAMLIO does report an
error if the yaml item is not a sequence. Instead, YAMLIO reads an
empty sequence. For example:

---
seq:
    foo: 1
    bar: 2
...

If `seq` is a SequenceTraits object, then reading the above yaml will
yield `seq` as an empty sequence.  The enclosed patch reports an error
for the above mapping ("not a sequence")

Null and empty nodes are still interpreted as empty sequences:

---
seq1:               # Empty node okay
---
seq2:  null       # null okay
...
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