Improve error message.

Improve error message.

... for ChannelError and APIMisuseError
Before this commit, When rl returns error, we don't know
The actual parameter which caused the error.
By returning parameterised `String` instead of predefined `&'static str`,
We can give a caller improved error message.

TestLogger now has two additional methods
1. `assert_log_contains` which checks the logged messsage
  has how many entry which includes the specified string as a substring.
2. `aasert_log_regex` mostly the same with `assert_log_contains`
  but it is more flexible that caller specifies regex which has
  to be satisfied instead of just a substring.
For regex, tests now includes `regex` as dev-dependency.