Improve error handling

[jonnor]

Most problematic is:

• no error feedback on custom component compilation/loading
• no error feedback on invalid input parameters (for instance images)
• no errors feedback on unexpected failures

Second one will require work in GEGL to do properly, as now this type of errors just eeks and prints to stdout/stderr somewhere.

[jonnor]

We have a bit of a mechanism for this now, sending the info across to Flowhub from imgflo_info/warning/critical calls. However from gegl etc was tricky due to disallowed recursion in g_log. Could be that custom handler which pushes things to a queue which is periodically checked and set to UI is way to go?