Add a "Clear last error" button to the API Tester

[joerg.hampel]

When using the API Tester to, well, test my module, and an error occurs, I always would like to be able to clear the error indicator before moving on - just to make my life easier by being able to see clearly when new, further errors occur. 

Like this:

Is there anything at all speaking against adding this nifty little feature? 

[Enrique.Noe]

I support this idea; it's hard to tell if the last error is happening again or if it's the same as one minute ago. Clearing this indicator will help us know whether what we're seeing is new.

[MichaelAivaliotis]

I already have this in my own DQMH template. It's the first thing I changed. So yes, I agree!

[joerg.hampel]

Accepted for next release of DQMH.