Add better exception msg for duplicated param names

[uriyyo]

Raise ValueError with information about duplicated names instead of sre_constants.error.

For instance:

app = Starlette(
    routes=[
        Route("/api/users/{id}/groups/{id}", route),
    ]
)

Will raise:

ValueError: Duplicated param name id at path /api/users/{id}/groups/{id}

Instead of:

sre_constants.error: redefinition of group name 'id' as group 2; was group 1 at position 42

[tomchristie]

Really nicely done. 👍