PARSE: error messages improved

Parse errors made more informative:

>> parse "abc" [1]
** Script error: PARSE - unexpected end of rule after: 1

>> parse "abc" [if]
** Script error: PARSE - unexpected end of rule after: if

>> parse "abc" [copy "x" "abc"]
** Script error: PARSE - expected a variable, not: "x"

>> parse "abc" [copy: "abc"]
** Script error: PARSE - command cannot be used as variable: copy:

>> bc: "bc"
>> parse "abc" ["a" :bc "bc"]
** Script error: PARSE - attempt to change input series: :bc

>> parse "abc" [if 1]
** Script error: PARSE - syntax error in rule: if

These should help users figure out rule problems.

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