Target panic message at lib users

Currently the panic message refers to stuff related to development of
the library, this is meaningless for users of the lib. Target panic
message at secp users instead.

src/key.rs

@@ -1053,7 +1053,7 @@ impl<'de> serde::Deserialize<'de> for KeyPair {
                 let ctx = Secp256k1::signing_only();
 
                 #[cfg(not(any(feature = "global-context", feature = "alloc")))]
-                let ctx: Secp256k1<crate::SignOnlyPreallocated> = panic!("The previous implementation was panicking too, please enable the global-context feature of rust-secp256k1");
+                let ctx: Secp256k1<crate::SignOnlyPreallocated> = panic!("cannot deserialize key pair without a context (please enable either the global-context or alloc feature)");
 
                 #[allow(clippy::needless_borrow)]
                 KeyPair::from_seckey_slice(&ctx, data)