Detecting errors in String operations

Hello,

If a String creation fails it is possible to detect this error in the code, like:

```C++
String s = "abc";
if (!s) ... // error, out of heap or something ...
```

However the error detection doesn't work with String expressions:

```C++
#define howLarge 5000
String *strArray = new String [howLarge];

void wasteAlmostAllMemory () {
for (int i = 0; i < howLarge; i++) strArray [i] = " wasted memory ";
while (true)
for (int i = 0; i < howLarge; i++)
if (!strArray [i].concat (" more wasted memory "))
return;
}

String returnLongString () {
return "This is a long string, much longer than fits into free memory, although not right now.";
}

void setup () {
Serial.begin (115200);

String longString = returnLongString ();

Serial.printf ("Wasting memory, please wait ... ");
wasteAlmostAllMemory ();
Serial.printf ("memory successfuly wasted\n");

String resultString;

// 1.
resultString = longString;
if (!resultString)
Serial.printf ("Could not create a long String s.\n"); // error detected successfuly
else { Serial.print ("'"); Serial.print (resultString); Serial.println ("'"); }

// 2.
resultString = "ABC " + longString + " DEF";
if (!resultString)
Serial.printf ("Could not calculate a String s.\n"); // error goes by undetected
else { Serial.print ("'"); Serial.print (resultString); Serial.println ("'"); } // the output is: ' DEF'

// 3.
resultString = returnLongString (); // failure to create the return String crashes the controller
}

void loop () {

}
```

My proposal is that each string expression that contains string in "error state" result in string also in "error state", so success could be tested only once even after several String operations. Something like this:

```C++
String s = "abc";
for (int i = 1; i < 1000; i++)
s += s.substring (1, 1);
if (!s) ... // error
```
Thank you.

El issue proporciona ejemplos de C++ en setup(), wasteAlmostAllMemory() y returnLongString(), pero no indica archivos de implementación ni tests. Empieza localizando la implementación de Arduino String y las rutas utilizadas para la concatenación, la asignación y los strings devueltos. Se considera terminado cuando las comprobaciones de fallo proporcionadas detectan los errores de forma consistente, sin salida parcial ni un fallo del controlador, y existe cobertura de regresión.

Escrito por el modelo de indexación a partir del texto del issue.