Fix DebugInfo being left in invalid state when loading fails

The pointer to internal AMX_DGB struct was not reset 0 in case of
dbg_LoadInfo() returning an error and thus DebugInfo was seen as "loaded"
from the outside (because is_loaded() simply checked amxdbg_ for nullness)
but actually it wasn't.
Fix dangling DebugInfo pointer in AmxLoad()
Improve debug info loading status message

- Leave only one of the two messages (no message == success)
- Remove redundant script name from the error message: it's printed on the next line anyway
- Append an error string that describes the error
Wrap ALL code in exported functions in try/catch for extra safety
Add missing "static"