Fix regression.
Fix regression.

SyntaxErrors are managed by the Engine, i.e. created
and must be cleared by the engine, as opposed to GraphErrors which
are cleared by the CellGraph.

stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila