I don't necessarily think that these are examples of mispronunciations. As @Roger Stoddard pointed out, the "shtr" pronunciation is probably present in a number of different dialects (and ideolects!). As such, I wouldn't call it a mispronunciation; rather, I see it as an example of dialectal difference, linguistic change, and linguistic variation.

I also see the addition of the "h" to the "str"consonant cluster as just another example of the linguistic devices that we employ all the time to ease pronunciation. My initial reaction is to call "shtr" an example of epenthesis: adding a sound to a word. But I could be wrong...