Don't raise exception if MIB init fails

[djanderson]

When using synthesized or very strong over-the-air input, the method of blindly assuming that frames passed to SSS & MIB blocks were valid was a fair assumption. However, when analyzing a heavily degraded signal, this assumption can lead the flowgraph to error out:

Expected subframe index 0, but got 5
Expected subframe index 0, but got 5
Error initiating PBCH
thread[thread-per-block[14]: <block mib (8)>]: Error initializing MIB
Expected subframe index 0, but got 5
OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO
>>> Done

We should be more tolerant of unexpected bad input by simply returning if MIB init fails, instead of throwing an error.

[djanderson]

This can still happen but seems to be mitigated by never using a psr_threshold below 1.5. All example applications use this convention but I should probably make it a hard-coded lower limit.