--maphost discards its own specific parse error

[korya]

Problem

parseHostMappings produces precise diagnostics:

• value %q has no separator, =
• value %q has no src port %q
• value %q has invalid src port %q
• value %q has invalid dst port %q

mustParseHostMappings throws all of them away and prints the raw slice instead.

Reproduction

$ http-assert --maphost 'garbage' --assert-ok http://127.0.0.1:8791/ok
Error: Invalid value for --maphost flag: [garbage]
[exit=71]

Expected something like:

Error: Invalid value for --maphost flag: value "garbage" has no separator, =

Affected code

main.go — mustParseHostMappings (the err return is ignored)

Suggested fix

Include err in the die format string. One-line change, free UX win.