Be more explicit with a I²C bus opening problem

firmware-updater/src/main.rs

@@ -16,6 +16,8 @@ mod elf_loader;
 
 #[derive(Debug, thiserror::Error)]
 enum Error {
+    #[error("cannot open I²C bus /dev/i2c-{bus}")]
+    OpenI2c { source: i2c::Error, bus: u8 },
     #[error(transparent)]
     RawI2c(#[from] i2c::Error),
     #[error(transparent)]
@@ -142,7 +144,10 @@ fn cmd_flash(args: &FlashArgs) -> Result<()> {
     }
 
     // Switch to bootloader mode
-    let mut i2c = I2c::with_bus(args.i2c_bus)?;
+    let mut i2c = I2c::with_bus(args.i2c_bus).map_err(|source| Error::OpenI2c {
+        source,
+        bus: args.i2c_bus,
+    })?;
     let program = if args.skip_firmware_version_check {
         ActiveProgram::Controller {
             version: Version::parse("0.0.0").unwrap(),