Add hint for UndefVarError string macro errors

[IanButterworth]

Master

julia> foo""
ERROR: LoadError: UndefVarError: `@foo_str` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
in expression starting at REPL[1]:1

This PR

julia> foo""
ERROR: LoadError: UndefVarError: `@foo_str` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Suggestion: You may be using the string macro form `foo""`, for which the expected `@foo_str` macro is not defined.
in expression starting at REPL[3]:1

todo:

• Tests
• Handle cmd macros too

julia> foo``
ERROR: LoadError: UndefVarError: `@foo_cmd` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
in expression starting at REPL[4]:1

[IanButterworth]

Suggested change

	print(io, "\nSuggestion: You may be using the string macro form `", m.captures[1], "\"\"`, for which the expected `$(var)` macro is not defined.")
	print(io, "\nSuggestion: You may be using the string macro form `", m.captures[1], "\"\"`, for which the necessary `$(var)` macro is not defined.")