Defaulter
/dɪˈfɔːltər/
Definition 1 of 2
Pro Tip 1/3Broken Promises
Refers to unmet obligations, not just unpaid debts. 
Since he didn't complete the work, he's a defaulter on the contract.
Pro Tip 2/3Financial Troubles
Often used to discuss serious money problems, not small mistakes. 
Her company became a defaulter after not repaying the massive loan.
Pro Tip 3/3Negative Connotation
Carries a negative judgement, implies irresponsible or unreliable behavior. 
As a habitual defaulter, he struggled to get any further credit.