clean up partial download, if program gets terminated during download

[pwolfart]

Sample console log, download cancelled using STRG+C

`0 [patrick@linux-lmem ~/workspace_git/imagizor (master)]$ ./imagizor.sh -d http://download.opensuse.org/distribution/leap/42.3/iso/openSUSE-Leap-42.3-DVD-x86_64.iso

---

download process and verifikation

---

Download the Software
--2017-11-20 10:11:44-- http://download.opensuse.org/distribution/leap/42.3/iso/openSUSE-Leap-42.3-DVD-x86_64.iso
Auflösen des Hostnamen »download.opensuse.org (download.opensuse.org)«... 2001:67c:2178:8::13, 195.135.221.134
Verbindungsaufbau zu download.opensuse.org (download.opensuse.org)|2001:67c:2178:8::13|:80... verbunden.
HTTP-Anforderung gesendet, warte auf Antwort... 302 Found
Platz: http://mirror.clarkson.edu/opensuse/distribution/leap/42.3/iso/openSUSE-Leap-42.3-DVD-x86_64.iso[folge]
--2017-11-20 10:11:44-- http://mirror.clarkson.edu/opensuse/distribution/leap/42.3/iso/openSUSE-Leap-42.3-DVD-x86_64.iso
Auflösen des Hostnamen »mirror.clarkson.edu (mirror.clarkson.edu)«... 128.153.145.19
Verbindungsaufbau zu mirror.clarkson.edu (mirror.clarkson.edu)|128.153.145.19|:80... verbunden.
HTTP-Anforderung gesendet, warte auf Antwort... 200 OK
Länge: 4639948800 (4,3G) [application/octet-stream]
In »»openSUSE-Leap-42.3-DVD-x86_64.iso«« speichern.

3% [====> ] 157.600.080 9,48MB/s ETA 7m 57s ^C
130 [p@linux~/workspace_git/imagizor (master)]$ ls
images imagizor imagizor.sh LICENSE openSUSE-Leap-42.3-DVD-x86_64.iso README.md
`

[pwolfart]

download will be continued on next call of the program