[devtools] Report error when source map URL not found in workspace

Also add url() accessor to WebInspector.SourceMap

BUG=302768

Review URL: https://codereview.chromium.org/25569003

git-svn-id: svn://svn.chromium.org/blink/trunk@159250 bbb929c8-8fbe-4397-9dbb-9b2b20218538