BUG: PNG IO: return false instead of crashing in CanReadFile
BUG: PNG IO: return false instead of crashing in CanReadFile

The other ImageIOs follow this principle in CanReadFile,
many of them having try/catch blocks to prevent crashing.

Example error message:

Exception detected while reading /mnt/scratch/dashboards/RHEL7-compact-gcc4.4-m32/ITK-build/Testing/Temporary/t1_z+_byte_cor.mnc : itk::ERROR: PNGImageIO(0x9e6fb18): PNGImageIO failed to read header for file:
Reason: fread read only 0 instead of 8

Change-Id: I02ecd7033fe8e8967d6a35e3c3670494736e6dca

InsightSoftwareConsortium/ITK,Kitware/ITK,blowekamp/ITK,vfonov/ITK,malaterre/ITK,malaterre/ITK,blowekamp/ITK,spinicist/ITK,spinicist/ITK,InsightSoftwareConsortium/ITK,BRAINSia/ITK,malaterre/ITK,spinicist/ITK,LucasGandel/ITK,fbudin69500/ITK,spinicist/ITK,malaterre/ITK,richardbeare/ITK,richardbeare/ITK,malaterre/ITK,thewtex/ITK,BRAINSia/ITK,thewtex/ITK,BRAINSia/ITK,LucasGandel/ITK,malaterre/ITK,richardbeare/ITK,malaterre/ITK,vfonov/ITK,richardbeare/ITK,Kitware/ITK,hjmjohnson/ITK,malaterre/ITK,fbudin69500/ITK,blowekamp/ITK,BRAINSia/ITK,stnava/ITK,spinicist/ITK,blowekamp/ITK,Kitware/ITK,LucasGandel/ITK,spinicist/ITK,BRAINSia/ITK,blowekamp/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,hjmjohnson/ITK,stnava/ITK,malaterre/ITK,fbudin69500/ITK,hjmjohnson/ITK,thewtex/ITK,LucasGandel/ITK,fbudin69500/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK,stnava/ITK,Kitware/ITK,stnava/ITK,blowekamp/ITK,fbudin69500/ITK,vfonov/ITK,fbudin69500/ITK,stnava/ITK,stnava/ITK,vfonov/ITK,hjmjohnson/ITK,thewtex/ITK,LucasGandel/ITK,stnava/ITK,blowekamp/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,LucasGandel/ITK,spinicist/ITK,thewtex/ITK,hjmjohnson/ITK,spinicist/ITK,BRAINSia/ITK,LucasGandel/ITK,hjmjohnson/ITK,vfonov/ITK,stnava/ITK,LucasGandel/ITK,fbudin69500/ITK,BRAINSia/ITK,Kitware/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,richardbeare/ITK,richardbeare/ITK,blowekamp/ITK,richardbeare/ITK,fbudin69500/ITK,Kitware/ITK,spinicist/ITK,thewtex/ITK,stnava/ITK