Show more precise error messages for invalid URLs
Show more precise error messages for invalid URLs

Be sure to include the field name in the error message.

Closes rust-lang/cargo#1490

rust-lang/crates.io,steveklabnik/crates.io,Susurrus/crates.io,Susurrus/crates.io,BlakeWilliams/crates.io,withoutboats/crates.io,steveklabnik/crates.io,Gankro/crates.io,Susurrus/crates.io,BlakeWilliams/crates.io,Susurrus/crates.io,chenxizhang/crates.io,BlakeWilliams/crates.io,steveklabnik/crates.io,withoutboats/crates.io,withoutboats/crates.io,achanda/crates.io,chenxizhang/crates.io,withoutboats/crates.io,Gankro/crates.io,achanda/crates.io,BlakeWilliams/crates.io,chenxizhang/crates.io,Gankro/crates.io,rust-lang/crates.io,chenxizhang/crates.io,rust-lang/crates.io,achanda/crates.io,steveklabnik/crates.io,achanda/crates.io,Gankro/crates.io,rust-lang/crates.io