Use nicer atoms for I2C bus errors

These errors can happen when the I2C bus hangs or a zero-byte transfer
is tried, but not supported. There are two kinds of timeouts:

* `{:error, :timeout}` - Seen on Allwinner when the I2C bus is hung by a
  device pulling SDA low. If the device tree/Linux isn't configured to
  recover and the SDA line isn't released, then the error will continue.
* `{:error, :retry}` - Seen on Beagleboards (TI AM335x) when the I2C bus
  is hung and there's an attempt to recover the bus.

Author: fhunleth

c_src/i2c_nif.c

@@ -116,6 +116,8 @@ struct I2cNifPriv {
 static ERL_NIF_TERM atom_ok;
 static ERL_NIF_TERM atom_error;
 static ERL_NIF_TERM atom_nak;
+static ERL_NIF_TERM atom_timeout;
+static ERL_NIF_TERM atom_retry;
 
 static void i2c_dtor(ErlNifEnv *env, void *obj)
 {
@@ -152,6 +154,8 @@ static int i2c_load(ErlNifEnv *env, void **priv_data, ERL_NIF_TERM info)
     atom_ok = enif_make_atom(env, "ok");
     atom_error = enif_make_atom(env, "error");
     atom_nak = enif_make_atom(env, "i2c_nak");
+    atom_timeout = enif_make_atom(env, "timeout");
+    atom_retry = enif_make_atom(env, "retry");
 
     *priv_data = priv;
     return 0;
@@ -173,10 +177,24 @@ static ERL_NIF_TERM enif_make_errno_error(ErlNifEnv *env)
         reason = atom_nak;
         break;
 #endif
+    case ETIMEDOUT:
+        // I2C bus hung. On some platforms, Linux can try to recover it.
+        reason = atom_timeout;
+        break;
+
+    case EAGAIN:
+        // I2C bus hung and an attempt is being made to recover it.
+        reason = atom_retry;
+        break;
+
     case ENOENT:
         reason = enif_make_atom(env, "bus_not_found");
         break;
 
+    case EOPNOTSUPP:
+        reason = enif_make_atom(env, "not_supported");
+        break;
+
     default:
         // strerror isn't usually that helpful, so if these
         // errors happen, please report or update this code