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.

该 issue 提供了 setup()、wasteAlmostAllMemory() 和 returnLongString() 中的 C++ 示例，但没有指明实现文件或测试。首先定位 Arduino String 的实现，以及用于拼接、赋值和返回字符串的路径。完成的标准是：所提供的失败检查能够稳定地检测错误，不产生部分输出或 controller 崩溃，并具有回归覆盖。

由索引模型根据 Issue 内容生成。