Error markers lost on save

[dragos]

This one is related to #84, but not the same. When saving a worksheet with errors, error markers may disappear completely:

• they may appear once, and disappear at a subsequent second save
• not appear at all

[dragos]

The race conditions is the following:

• on the evaluation thread, the editor contents is stripped and the document contents is replaced completely with the stripped contents. This triggers a background job on the platform to remove markers whose positions are affected by the change (meaning all of them, since we replace the whole contents)
• the compilation errors are reported and markers are created

Depending which thread is faster, you may see or not the compilation errors.