Make test-error handle compound conditions properly
Attach load context info to errors occurred during loading
Fix inifinite loop bug in (slot-missing <condition-meta> ...)
Add Scm_InitializedP()

To check whether we finished initializing Scheme infrastructure
Rewrite part of error reporting in Scheme, generalize mixin conditions

Error messages are now constructed in Scheme routine, which opens
opportunities for better error message generation.