bug: CRC could be lowercase, which does not work

[ninegua]

The python oneliner in README computes CRC, but if the result contains lower-case letter, it will not be recognized as a valid CanisterId, which leads to the following error when visiting the page in browser:

An error happened:
fromText@http://127.0.0.1:8080/index.js:2:21406
query@http://127.0.0.1:8080/index.js:2:142932
retrieveAsset@http://127.0.0.1:8080/index.js:2:143045
_loadJs@http://127.0.0.1:8080/index.js:2:89546
_main@http://127.0.0.1:8080/index.js:2:90616
@http://127.0.0.1:8080/index.js:2:91079
r@http://127.0.0.1:8080/index.js:2:358
@http://127.0.0.1:8080/index.js:2:1157
@http://127.0.0.1:8080/index.js:2:1167
@http://127.0.0.1:8080/index.js:2:158
@http://127.0.0.1:8080/index.js:2:223

[ninegua]

I also think we need better error message for an invalid CanisterId. Probably something in the userlib is not handling the response correctly. @hansl