parser: corrected reporting of errors in numbers

there was an error condition that was not detected properly, which was
reported as "expected sign or digits for exponent in floating point
constant" but an exponent sign was present, so a new "expected digits
for exponent in floating point constant" error was added and the get
number function was modified to detect and throw this new error

changed the "expected digits in mantissa of floating point constant"
error was to point to both the '.' and 'E' in the incorrect ".E"

changed translator to request a reference token for the first token
added reference argument (default none) to translator get token
  (prevent number tokens when reference is not none)
changed translator get operand to pass reference flag to get token
  (which now reports appropriate error, so upon error just return status)

changed the "expected command" error to say "expected command or
item for assignment" (status enumerator also changed)

cleaned up the let translate error handling

updated parser test 3 (number tests) for changes in error messages
updated translator tests 1, 3 (assignments) and 14 for error changes
added few more tests to translator test 14 (parser) to test new error
updated several translator tests for expected command error change