Fix error handling/reporting for "adb forward" and "adb reverse".

We really need better infrastructure for parsing adb subcommands, but
in the meantime...

At least this cleans up a little more of the implementation too.

Bug: http://b/20736014
Change-Id: I76209847da3724906c71924017bcb69fa31e0b49
diff --git a/adb/adb_client.h b/adb/adb_client.h
index 9895c49..5de0638 100644
--- a/adb/adb_client.h
+++ b/adb/adb_client.h
@@ -26,9 +26,9 @@
 int adb_connect(const std::string& service, std::string* error);
 int _adb_connect(const std::string& service, std::string* error);
 
-// Connect to adb, connect to the named service, return 0 if the connection
-// succeeded AND the service returned OKAY.
-int adb_command(const std::string& service, std::string* error);
+// Connect to adb, connect to the named service, returns true if the connection
+// succeeded AND the service returned OKAY. Outputs any returned error otherwise.
+bool adb_command(const std::string& service);
 
 // Connects to the named adb service and fills 'result' with the response.
 // Returns true on success; returns false and fills 'error' on failure.