Pyramidal Strings

In this challenge, you have to obtain a pyramidal version of a given string, transforming the string into a list containing a series of string slices that progressively increase or decrease steadily from the left to the right. Every slice containing two or more characters must have a space between every pair of characters, to permit a hypothetical vertical alignment. See the example below:

# REGULAR pyramidal version of "INNOKODAKADEMIJA"

[ "E",
 "D A",
"B I T" ]

Depending on the given _type, you have to obtain a regular pyramid starting from its vertex (_type === "REG") as in the example above, or a reversed pyramid starting from its base (_type === "REV") as in the example below:

# REVERSED pyramidal version of "INNOKODAKADEMIJA"

["E D A",
  "B I",
   "T"  ]

Every pyramid must follow the same steadily increment/decrement of its slices (or rows) by exactly one character (excluding spaces), so that not every string can be transformed in a pyramid! See the example below:

# Regular pyramidal version of "PYRAMID"

[ "P",
 "Y R",
"A M I" ]

# Letter "D" can't be placed in the pyramid

Given as parameters a string and a _type, implement a function that returns:

• A string "Error!" if the pyramidal version can't be obtained from the given string.
• A list containing the regular pyramidal version of the string if the given _type is equal to "REG".
• A list containing the reversed pyramidal version of the string if the given _type is equal to "REV".

Examples

pyramidal_string("INNOKODAKADEMIJA", "REG") ➞ ["E", "D A", "B I T"]

pyramidal_string("INNOKODAKADEMIJA", "REV") ➞ ["E D A", "B I", "T"]

pyramidal_string("PYRAMID", "REG") ➞ "Error!"

pyramidal_string("!", "REV") ➞ ["!"]

pyramidal_string("", "REG") ➞ []

Notes

• If the given string has just one character, the returned list will contain that single character. If the given string is empty, the returned list will be empty.
• Remember to insert a space between every character inside the rows containing two or more characters.
• The increment and the decrement of the rows in a pyramidal string are equal to one character more or less than the previous, depending on the given _type.
• You have to find a discriminant rule to establish if a string can be transformed into a pyramid, without creating single exceptions for every given case. What is suggesting to you the shape of a pyramid seen frontally?