Clarified exception

src/objective-c/GRPCClient/GRPCCall+Tests.m

@@ -27,8 +27,14 @@
 + (void)useTestCertsPath:(NSString *)certsPath
                 testName:(NSString *)testName
                  forHost:(NSString *)host {
-  if (!host || !certsPath || !testName) {
-    [NSException raise:NSInvalidArgumentException format:@"host, path and name must be provided."];
+  if (!host) {
+    [NSException raise:NSInvalidArgumentException format:@"host must be provided."];
+  }
+  if (!certsPath) {
+    [NSException raise:NSInvalidArgumentException format:@"certpath be provided."];
+  }
+  if (!testName) {
+    [NSException raise:NSInvalidArgumentException format:@"testname must be provided."];
   }
   NSError *error = nil;
   NSString *certs =