refactor __init__ functions of Wallets (funding sources)

- better error messages when a conf variable is not provided
- unify approaches among different source files

lnbits/wallets/lnbits.py

@@ -20,14 +20,20 @@ class LNbitsWallet(Wallet):
     """https://github.com/lnbits/lnbits"""
 
     def __init__(self):
-        self.endpoint = settings.lnbits_endpoint
+        if not settings.lnbits_endpoint:
+            raise ValueError("cannot initialize LNbitsWallet: missing lnbits_endpoint")
         key = (
             settings.lnbits_key
             or settings.lnbits_admin_key
             or settings.lnbits_invoice_key
         )
-        if not self.endpoint or not key:
-            raise Exception("cannot initialize lnbits wallet")
+        if not key:
+            raise ValueError(
+                "cannot initialize LNbitsWallet: "
+                "missing lnbits_key or lnbits_admin_key or lnbits_invoice_key"
+            )
+        endpoint = settings.lnbits_endpoint
+        self.endpoint = endpoint[:-1] if endpoint.endswith("/") else endpoint
         self.headers = {"X-Api-Key": key, "User-Agent": settings.user_agent}
         self.client = httpx.AsyncClient(base_url=self.endpoint, headers=self.headers)