Do not include a backtick in error messages and backtraces

[mame]

[Feature #16495]

TODO:

• Need to update prism's error messages.
• Need to fix ruby/spec too.
• By changing keyword display name from `true' to 'true', Lrama adds unneeded double quotes. @yui-knk fixed

$ ./miniruby -e 'def foo(true)'
-e:1: syntax error, unexpected "'true'", expecting ')'
def foo(true)
-e: compile error (SyntaxError)

$ ./miniruby -e 'foo(1 rescue 2)'
-e:1: syntax error, unexpected "'rescue' modifier", expecting ')'
foo(1 rescue 2)
-e: compile error (SyntaxError)