Improve Blob constructor error messages.

Follow current practice of supplying the name of the constructor in
the error message string, along with normalizing formatting and
wording used.

Also took the opportunity of simplifying how toV8Sequence() reports a
type error: a non-sequence argument is now indicated by returning
v8Undefined().

R=ch.dumez@samsung.com
BUG=270033

Review URL: https://codereview.chromium.org/68533014

git-svn-id: svn://svn.chromium.org/blink/trunk@161849 bbb929c8-8fbe-4397-9dbb-9b2b20218538