Improved exception when not running as root (#564)

When RF24 wasn't ran as root, it threw a very cryptic error (see #531): `terminate called after throwing an instance of 'int'`

This commit will improve that and now it appears as: `terminate called after throwing an instance of 'std::runtime_error' what():  Process should run as root`

utility/RPi/spi.cpp

@@ -1,6 +1,7 @@
 #include "spi.h"
 #include <pthread.h>
 #include <unistd.h>
+#include <stdexcept>
 
 static pthread_mutex_t spiMutex = PTHREAD_MUTEX_INITIALIZER;
 bool bcmIsInitialized = false;
@@ -24,7 +25,7 @@ void SPI::begin(int busNo)
 void SPI::beginTransaction(SPISettings settings)
 {
     if (geteuid() != 0) {
-        throw -1;
+        throw std::runtime_error("Process should run as root");
     }
     pthread_mutex_lock(&spiMutex);
     setBitOrder(settings.border);