CertificateToString() ignores PEM_write_bio_X509() error

[Al2Klimov]

Describe the bug

CertificateToString() calls PEM_write_bio_X509() to write a cert as PEM into memory (BIO_new(BIO_s_mem())) and ignores any errors:

• "Writes to memory BIOs will always succeed if memory is available" (bio_s_mem(3)), so this by itself shouldn't^TM be a problem unless OOM. But even in that enge case, I'd prefer an exception instead of a pseudo-successfully returned truncated result.
• Also, something could theoretically be wrong with the certificate.

To Reproduce

icinga2/lib/base/tlsutility.cpp

Lines 802 to 805 in fe85bf1

	String CertificateToString(X509* cert)
	{
	BIO *mem = BIO_new(BIO_s_mem());
	PEM_write_bio_X509(mem, cert);

Expected behavior

If PEM_write_bio_X509() fails, throw an exception, just like MakeX509CSR().

Additional context

Random finding during:

ref/IP/57407