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.

この issue には setup()、wasteAlmostAllMemory()、returnLongString() の C++ 例が示されていますが、実装ファイルやテストは指定されていません。まず Arduino String の実装と、連結、代入、返される文字列に使用されているパスを見つけてください。提供された失敗チェックが部分的な出力やコントローラーのクラッシュなしにエラーを一貫して検出し、回帰テストのカバレッジがあることが完了の条件です。

索引モデルが issue の本文から書いたものです。