[lang] Improve invalid selector error

[cmichi]

Previously it was just:

$ cargo build                                                                                                                                                  
error: custom attribute panicked                                                                                                                               
  --> lib.rs:19:1                                                                                                                                              
   |                                                                                                                                                           
19 | #[ink::contract]                                                                                                                                          
   | ^^^^^^^^^^^^^^^^                                                                                                                                          
   |                                                                                                                                                           
   = help: message: called `Option::unwrap()` on a `None` value                                                                                                
                                                                                                                                                               
error: aborting due to previous error  

[ascjones]

LGTM

[codecov-io]

Codecov Report

Merging #561 into master will increase coverage by 2.31%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #561      +/-   ##
==========================================
+ Coverage   80.22%   82.53%   +2.31%     
==========================================
  Files         155      155              
  Lines        6811     6820       +9     
==========================================
+ Hits         5464     5629     +165     
+ Misses       1347     1191     -156     

Impacted Files	Coverage Δ
crates/lang/ir/src/ir/attrs.rs	85.58% <75.00%> (+0.14%)	⬆️
crates/env/src/engine/off_chain/chain_extension.rs	60.00% <0.00%> (-40.00%)	⬇️
crates/env/src/engine/off_chain/db/chain_spec.rs	80.00% <0.00%> (-20.00%)	⬇️
crates/env/src/engine/off_chain/mod.rs	77.92% <0.00%> (-11.69%)	⬇️
crates/env/src/engine/off_chain/db/console.rs	15.78% <0.00%> (-10.53%)	⬇️
crates/env/src/engine/off_chain/db/accounts.rs	75.32% <0.00%> (-10.39%)	⬇️
crates/env/src/engine/off_chain/db/events.rs	9.67% <0.00%> (-6.46%)	⬇️
crates/lang/ir/src/ir/trait_def.rs	92.82% <0.00%> (+9.74%)	⬆️
crates/lang/ir/src/ir/item_impl/constructor.rs	98.30% <0.00%> (+13.55%)	⬆️
... and 10 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c85cb78...809fb97. Read the comment docs.

[Robbepop]

LGTM

Thanks a lot for the highly anticipated error reporting improvements! :)