If "errored" is not a valid word, then how should I say:
The program errored at line 44
I guess I could say:
The program threw an error at line 44
But why is "errored" wrong? Is there a better alternative?
You can actually say "The program erred at line 44", but it's not very idiomatic. "Err" also occurs in the saying "To err is human, to forgive is divine".
If you're looking for the correct idiom, you could say "the program encountered an error at line 44" or "the program hit an error at line 44", etc.
I'd say errored IS a valid word. It's the past tense of the verb "to error". I've seen (well, mostly heard) this word used to mean
This is a relatively recent usage of the word (I can't find any authoritative samples of it) it might be considered too informal or slangy. Also, some people might not be sure what precisely you are trying to say. Thus, you should describe more fully what the program is doing.
The program encountered an error at line 44.
The verb 'to error' has a different meaning than 'to err'. An "error" in a computer program isn't necessarily a mistake, but can be an exceptional circumstance. For example, if a program tried to open its configuration file, but you deleted it, the program might fail by displaying an error for this unexpected circumstance. You could say "the program errored." You can't say "the program erred" because the program isn't making a mistake here.
Within programming circles, I'd say that "errored" is a perfectly fine term to use. It seems to be fairly widely understood as a verb form for "error". Outside of computing, I'd probably avoid it. You generally won't hear someone say "Todd errored on his test". It's a field-specific bit of terminology.
That being said, while discussing things within the field of computing, I'd prefer "the program errored/failed/crashed" over "erred", as some others have suggested. Any of those would be concise and make sense.
I would also prefer the options above over "The program threw an error" in informal settings, simply for the fact that it's less wordy, unless the focus of the statement was how exactly the program did the throwing. In technical documents, "threw an error" might be preferable to "errored".