Add -Werror to acceptance builds for C and CPP (#4369)

Use platform.local.txt to add -Werror to GCC for the build of all
code.  Any warnings on a submitted patch will cause an error.

Several examples and libraries had warnings/errors (missing returns
on functions, types, etc.).  Clean those up with this commit as well.

libraries/Ethernet/examples/BarometricPressureWebServer/BarometricPressureWebServer.ino

@@ -204,7 +204,7 @@ unsigned int readRegister(byte registerName, int numBytes) {
   // take the chip select low to select the device:
   digitalWrite(chipSelectPin, LOW);
   // send the device the register you want to read:
-  int command = SPI.transfer(registerName);
+  SPI.transfer(registerName);
   // send a value of 0 to read the first byte returned:
   inByte = SPI.transfer(0x00);