Remove fail fast matches

Removed most fail fast pattern matches in an effort to not crash and provide a better developer experience.

Added rebar3_hex_config:get_hex_config/3
Added rebar3_hex_error:format_error/1 clauses per get_hex_config/3
Removed hex_config/2, hex_config_write/1, and hex_config_read/1 from rebar3_hex_config exports
Removed existing helpers and obsoloete format_error/1 clauses on providers in favor of rebar3_hex_config:get_hex_config/3

erlef / rebar3_hex