Fix Maven download URL and improve logs

The Maven URL to download a package was wrong when the package groupID had
dots inside. We must replace them by `/`.

For example, without this MR, to download the package
`javax.servlet/servlet-api`, the URL used was:
    - https://repo1.maven.org/maven2/javax.servlet/servlet-api/3.0-alpha-1/servlet-api-3.0-alpha-1.jar

But, if tested using curl:
```
$ curl --head https://repo1.maven.org/maven2/javax.servlet/servlet-api/3.0-alpha-1/servlet-api-3.0-alpha-1.jar
HTTP/2 404
...
```

The right URL is:
    - https://repo1.maven.org/maven2/javax/servlet/servlet-api/3.0-alpha-1/servlet-api-3.0-alpha-1.jar

And if tested with curl:
```
$ curl --head https://repo1.maven.org/maven2/javax/servlet/servlet-api/3.0-alpha-1/servlet-api-3.0-alpha-1.jar
HTTP/2 200
...
x-checksum-md5: fe58aea60c6acf7cdf79302d568bc9a8
x-checksum-sha1: 7b491267924e8c81f4f9378caba0ee03423948ca
...
```

This MR also improves logs.

Found while working on it/package-registry#27

Author: leocardao

src/e3/maven.py

@@ -21,7 +21,10 @@ def __init__(self, group: str, name: str, version: str) -> None:
         self.package_group = group
         self.package_name = name
         self.version = version
-        self.url = f"https://repo1.maven.org/maven2/{group}/{name}/{version}/{filename}"
+        self.url = (
+            f"https://repo1.maven.org/maven2/{group.replace('.', '/')}/{name}/{version}"
+            f"/{filename}"
+        )
 
         # To get the expected checksum of the current file, we need to make an
         # additonnal HEAD request. This is because maven send the checksum directly on
@@ -32,7 +35,7 @@ def __init__(self, group: str, name: str, version: str) -> None:
         sha1_checksum = hdrs.get("x-checksum-sha1")
         md5_checksum = hdrs.get("x-checksum-md5")
         if not md5_checksum and not sha1_checksum:
-            raise RuntimeError("No checksum provided")
+            raise RuntimeError(f"No checksum provided for {group}/{self.filename}")
 
         # No 'elif' because maven can send both together into HTTP headers.
         if md5_checksum: