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.

La issue fornisce esempi C++ in setup(), wasteAlmostAllMemory() e returnLongString(), ma non indica file di implementazione né test. Inizia individuando l’implementazione di Arduino String e i percorsi utilizzati per la concatenazione, l’assegnazione e le stringhe restituite. Il lavoro è completato quando i controlli di errore forniti rilevano gli errori in modo coerente, senza output parziale né crash del controller, con copertura di regressione.

Scritto dal modello di indicizzazione a partire dal testo della issue.