isa: DANGLING:123 causes parse error

We have the following problem in the latest Mondo release:

```
[Term]
id: MONDO:0021125
name: disease characteristic
def: "An attribute of a disease." [https://orcid.org/0000-0002-6601-2165]
synonym: "disease qualifier" EXACT []
synonym: "modifier" EXACT [NCIT:C41009]
synonym: "qualifier" EXACT [NCIT:C41009]
xref: NCIT:C41009 {source="MONDO:equivalentTo"}
is_a: PATO:0000001
property_value: exactMatch NCIT:C41009
```

When running:

```
runoak --input pronto:$< info MONDO:0000001
```

is causing:

```
KeyError: 'PATO:0000001'
```

When running:

```
fastobo-validator mondo.obo
Parsing `mondo.obo`
Finished parsing `mondo.obo` in 0.73s
Completed validation of `mondo.obo`
```

Everything is all good.

When removing the isa statement above:

```
[Term]
id: MONDO:0021125
name: disease characteristic
def: "An attribute of a disease." [https://orcid.org/0000-0002-6601-2165]
synonym: "disease qualifier" EXACT []
synonym: "modifier" EXACT [NCIT:C41009]
synonym: "qualifier" EXACT [NCIT:C41009]
xref: NCIT:C41009 {source="MONDO:equivalentTo"}
property_value: exactMatch NCIT:C41009
```

Everything is good as well:

```
runoak --input pronto:mondo.obo info MONDO:0000001
MONDO:0000001 ! disease
```

As there are thousands of dangling classes in mondo.obo - what seems to be the problem?