Make the errors mapping table entries less cryptic

[kennystrawnmusic]

I've spent hours debugging the following panic message thrown when a value larger than the 8 bits defined in the timer and error vector constants (I was trying to put 0x320 in the timer vector field and 0x280 in the error vector field per what's specified on osdev.org) is plugged into the builder functions:

assertion failed: `(left == right)`
left: `3`
right: `0`

This message is coming from the bit crate, which performs an assert_eq! check to ensure that the bits are of the same length. Yet, it does absolutely nothing to inform the developer of what is being done wrong. So, I'm submitting this pull request to save other people the trouble by printing a more legible error message.

[kwzhao]

Thanks @kennystrawnmusic!