loader: Correct overly strict startup checks.

The code which handled loading modules had too many situations which
would result in halting Asterisk startup.  Treat most errors as declines
instead of failures.  The exception is when the module load function
returns AST_MODULE_LOAD_FAILURE or an invalid code.

Clear the missingdeps vector when appropriate to ensure the next loop
starts clean.

ASTERISK-27620

Change-Id: I45547d9641fd45bd86d80250224417625631ad84