Better error handling

[LordAro]

Currently, if you make an error in the mermaid text, the error is only displayed in the devtools console, which isn't particularly user friendly.

With #69 & 8.10.2, it appears to have gotten worse, with no console message and only the following image being displayed:

which is even less helpful.

Can we get a better way of displaying errors?

The mermaid docs mention a parseError function, which seems designed for this purpose - https://mermaid-js.github.io/mermaid/#/usage?id=advanced-usage . I had a brief play, but couldn't get it to work with the existing initialize & init workflow in ext.mermaid.js (which appears to be deprecated anyway, according to the documentation?)

[RNitzsche]

Thanks for the mermaid extension.

This has gotten even more worse in the current version. The image is displayed in den foot of the page.

Having serveral mermaids on one page, I've to decide which one to debug.