Fix nasa#482, used return value of backtrace in OS_printf() statement

[myc-yang]

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Describe the contribution
Fixed #482 by using return value of backtrace in OS_printf() statement

Testing performed

Expected behavior changes

• A message will be printed on the screen when running the pc-linux psp whenever you are attaching an exception.

System(s) tested on

• Hardware: PC
• OS: Ubuntu 18.04
• Versions: [e.g. cFE 6.6, OSAL 4.2, PSP 1.3 for mcp750, any related apps or tools]

Additional context
Add any other context about the contribution here.

Third party code
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Contributor Info - All information REQUIRED for consideration of pull request
Michael Yang NASA/GSFC