FuelGauge: return error if it failed to read/write registers.

[XuGuohui]

Problem

Returned value of FuelGauge's wiring APIs cannot indicate error on failed.

Steps to Test

1. Build and flash the following example application for B5SoM that is attached on SoM evaluation board.
2. Connect the SoM evaluation board to serial terminal.
3. Detach the jumper for PM_SCL or PM_SDA and see if a negative value is printed on serial terminal.

Example App

#include "Particle.h"

SYSTEM_MODE(MANUAL);

SerialLogHandler log(115200, LOG_LEVEL_ALL);

void setup()
{
    Serial.begin(115200);
    while(!Serial.isConnected());
    LOG(TRACE, "Application started.");
}

void loop() {
    FuelGauge gauge;
    LOG(TRACE, "SoC: %4.2f %%", gauge.getSoC());
    delay(1000);
}

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)