collect/ endpoint query for conan PURL with non-existent version returns 'Server Error (500)'

Using the local `collect/` endpoint to populate my local DB, if I search for a `conan` type with a version that does not exist, I get a `Server Error (500)`. This does not happen with at least some of the other supported types in the [CollectViewSet() class list](https://github.com/aboutcode-org/purldb/blob/main/packagedb/api.py#L980-L989).

- For example, according to the [ConanCenter](https://conan.io/center/recipes/alpaca), the only version of alpaca is 0.2.1, and a query for 0.3.0 (`/api/collect/?purl=pkg:conan/alpaca@0.3.0`) throws the error.

- OTOH, [NPM](https://www.npmjs.com/package/log4js) tells me the latest version for log4j is 6.9.1, and a query for 7.0.0 (`/api/collect/?purl=pkg:npm/log4js@7.0.0`) returns a JSON object informing me that the package does not exist.

```
GET /api/collect/?purl=pkg:npm/log4js@7.0.0

HTTP 400 Bad Request
Allow: GET, HEAD, OPTIONS
Content-Type: application/json
Vary: Accept

{
"status": "error(s) occurred when fetching metadata for pkg:npm/log4js@7.0.0: Package does not exist on npmjs: pkg:npm/log4js@7.0.0"
}
```

- Similar result for the [MVN Repository](https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind), which tells me the latest version of Jackson Databind is 2.17.2 and responds to a `collect/` query for 2.20.0 with

```
HTTP 400 Bad Request
Allow: GET, HEAD, OPTIONS
Content-Type: application/json
Vary: Accept

{
"status": "error(s) occurred when fetching metadata for pkg:maven/com.fasterxml.jackson.core/jackson-databind@2.20.0: Package does not exist on maven: pkg:maven/com.fasterxml.jackson.core/jackson-databind@2.20.0\nPackage does not exist on maven: pkg:maven/com.fasterxml.jackson.core/jackson-databind@2.20.0?classifier=sources\n"
}
```